WEBVTT captioned by sachac

NOTE Introduction

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Hello everyone, my name is Ihor Radchenko,

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and you may know me from Org Mailing List.

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However, today I'm not going to talk about Org Mode.

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Today I'm going to talk about

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Emacs performance and how it's affected

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by its memory management code.

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First, I will introduce the basic concepts

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of Emacs memory management and what garbage collection is.

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Then I will show you user statistics

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collected from volunteer users over the last half year

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and I will end with some guidelines

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on how to tweak Emacs garbage collection customizations

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to optimize Emacs performance

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and when it's necessary or not to do.

NOTE About garbage collection in Emacs

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Let's begin. What is garbage collection?

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To understand what is garbage collection,

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we need to realize that anything you do in Emacs

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is some kind of command. Any command is most likely

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running some Elisp code. Every time you run Elisp code,

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you most likely need to locate certain memory in RAM.

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Some of this memory is retained for a long time

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and some of this memory is transient.

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Of course, Emacs has to clear this transient memory

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from time to time, to not occupy all the possible RAM

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in the computer. In this small example,

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we have one global variable

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that is assigned a value,

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but when assigning the value,

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we first allocate a temporary variable

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and then a temporary list

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and only retain some part of this list

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in this global variable.

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In terms of memory graph

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we can represent this as two variable slots,

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one transient, one permanent,

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and then a list of three cons cells,

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part of which is retained as a global variable

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but part of it which is a temporary variable symbol.

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The first term of the list is not used

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and it might be cleared at some point.

NOTE Garbage collection in Emacs

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So that's what Emacs does.

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Every now and then, Emacs goes through all the memory

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and identifies which part of the memory are not used

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and then clear them so that it can free up the RAM.

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This process is called garbage collection

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and Emacs uses a very simple and old algorithm

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which is called Mark & Sweep.

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So doing this mark and sweep process

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is basically two stages.

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First, Emacs scans all the memory that is allocated

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and then identifies which memory is still in use

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which is linked to some variables, for example,

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and which memory is not used anymore

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even though it was allocated in the past.

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The second stage [??] whenever a memory is not,

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that is not allocated. During the process

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Emacs cannot do anything now.

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So basically, every time Emacs scans the memory,

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it freezes up and doesn't respond to anything,

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and if it takes too much time so that users can notice it,

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then of course Emacs is not responsive at all,

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and if this garbage collection is triggered too frequently,

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then it's not just not responsive every now and then.

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It's also not responsive all the time,

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almost all the time,

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so it cannot even normally type or stuff

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or do some normal commands.

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This mark and sweep algorithm is taking longer

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the more memory Emacs uses. So basically,

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the more buffers you open, the more packages you load,

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the more complex commands you run, the more memory is used,

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and basically, the longer Emacs takes

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to perform a single garbage collection.

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Of course, Emacs being Emacs

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this garbage collection can be tweaked.

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In particular users can tweak

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how frequently Emacs does garbage collection

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using two basic variables: `gc-cons-threshold`

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and `gc-cons-percentage`.

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`gc-cons-threshold` is the raw number of kilobytes

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Emacs needs to allocate

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before triggering another garbage collection,

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and the `gc-cons-percentage` is similar,

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but it's defined in terms of fraction

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of already-allocated memory.

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If you follow various Emacs forums,

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you may be familiar with people complaining about

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garbage collection. There are many many suggestions

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about what to do with it.

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Most frequently, you see `gc-cons-threshold`

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recommended to be increased,

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and a number of pre-packaged Emacs distributions

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like Doom Emacs do increase it.

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I have seen suggestions which are actually horrible

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to disable garbage collection temporarily

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or for a long time.

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Which is nice... You can see it quite frequently,

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which indicates there might be some problem.

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However, every time one user poses about this problem,

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it's just one data point and it doesn't mean

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that everyone actually suffers from it.

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It doesn't mean that everyone should do it.

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So in order to understand if this garbage collection

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is really a problem which is a common problem

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we do need some kind of statistics

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and only using the actual statistics

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we can understand if it should be recommended for everyone

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to tweak the defaults or like whether

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it should be recommended for certain users

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or maybe it should be asked Emacs devs

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to do something about the defaults.

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And what I did some time ago is exactly this.

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I tried to collect the user statistics.

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So I wrote a small package on Elp

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and some users installed this package

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and then reported back these statistics

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of the garbage collection for their particular use.

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By now we have obtained 129 user submissions

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with over 1 million GC records in there.

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So like some of these submissions

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used default GC settings without any customizations.

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Some used increased GC cost threshold

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and GC cost percentage.

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So using this data we can try to draw

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some reliable conclusions on what should be done

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and whether should anything be done about garbage collection

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on Emacs dev level or at least on user level.

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Of course we need to keep in mind

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that there's some kind of bias

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because it's more likely

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that users already have problems with GC

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or they think they have problems with GC

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will report and submit the data.

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But anyway having s statistics is much more useful

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than just having anecdotal evidences

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from one or other reddit posts.

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And just one thing I will do

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during the rest of my presentation

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is that for all the statistics

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I will normalize user data

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so that every user contributes equally.

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For example if one user submits like

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100 hours Emacs uptime statistics

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and other users submit one hour Emacs uptime

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then I will anyway make it so that they contribute equally.

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Let's start from one of the most obvious things

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we can look into is

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which is the time it takes for garbage collection

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to single garbage collection process.

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Here you see frequency distribution of GC duration

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for all the 129 users we got

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and you can see that most of the garbage collections

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are done quite quickly in less than 0.1 second

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and less than 0.1 second is usually just not noticeable.

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So even though there is garbage collection

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it will not interrupt the work in Emacs.

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However there is a fraction of users

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who experience garbage collection

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it takes like 0.2, 0.3 or even half a second

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which will be quite noticeable.

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For the purposes of this study

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I will consider that anything that is less than 0.1 second

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which is insignificant so like you will not notice it

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and it's like obviously

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all the Emacs usage will be just normal.

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But if it's more than 0.1 or 0.2 seconds

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then it will be very noticeable

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and you will see that Emacs hang for a little while

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or not so little while. In terms of numbers

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it's better to plot the statistics not as a distribution

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but as a cumulative distribution.

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So like at every point of this graph

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you'll see like for example here 0.4 seconds

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you have this percent of like almost 90% of users

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have no more than 0.4 gc duration.

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So like we can look here if we take one

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gc critical gc duration which is 0.1 second

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0.1 second and look at how many users have

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it so we have 56% which is like

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44% users have less than 0.1 second gc duration

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and the rest 56% have more than 0.1 second.

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So you can see like more than half of users

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actually have noticeable gc delay

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so the Emacs freezes for some noticeable time

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and a quarter of users actually have very noticeable

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so like Emacs freezes such that you see an actual delay

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that Emacs actually has

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which is quite significant and important point.

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But apart from the duration of each individual gc

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it is important to see how frequent it is

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because even if you do notice a delay

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even a few seconds delay

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it doesn't matter if it happens once

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during the whole Emacs session.

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So if you look into frequency distribution again here

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I plot time between subsequent garbage collections

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versus how frequent it is and we have very clear trend

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that most of the garbage collections are quite frequent

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like we talk about every few seconds a few tens of seconds.

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There's a few outliers which are at very round numbers

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like 60 seconds, 120 seconds, 300 seconds.

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These are usually timers so like

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you have something running on timer

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and then it is complex command

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and it triggers garbage collection

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but it's not the majority.

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Again to run the numbers

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it's better to look into cumulative distribution

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and see that 50% of garbage collections

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are basically less than 10 seconds apart.

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And we can combine it with previous data

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and we look into whatever garbage collection

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takes less than 10 seconds from each other

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and also takes more than say 0.1 seconds.

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So and then we see that

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one quarter of all garbage collections

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are just noticeable and also frequent

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and 9% are not like

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more than 0.2% very noticeable and also frequent.

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So basically it constitutes Emacs freezing.

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So 9% of all the garbage collection Emacs freezing.

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Of course if you remember there is a bias

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but 9% is quite significant number.

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So garbage collection can really slow down things

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not for everyone but for significant fraction of users.

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Another thing I'd like to look into

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is what I call agglomerated GCs.

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What I mean by agglomerated is

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when you have one garbage collection

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and then another garbage immediately after it.

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So in terms of numbers I took

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every subsequent garbage collection

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which is either immediately after

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or no more than one second after each.

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So from point of view of users is like

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multiple garbage collection they add up together

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into one giant garbage collection.

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And if you look into numbers

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of how many agglomerated garbage collections there are

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you can see even numbers over 100.

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So 100 garbage collection going one after another.

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Even if you think about each garbage collection

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taking 0.1 second we look into 100 of them

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it's total 10 seconds.

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It's like Emacs hanging forever

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or like a significant number is also 10.

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So again this would be very annoying to meet such thing.

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How frequently does it happen?

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Again we can plot cumulative distribution

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and we see that 20 percent like 19 percent

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of all the garbage collection are at least two together

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and 8 percent like more than 10. So like you think about oh

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each garbage collection is not taking much time

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but when you have 10 of them yeah that becomes a problem.

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Another thing is to answer a question

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that some people complain about is that

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longer you use Emacs the slower Emacs become.

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Of course it may be caused by garbage collection

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and I wanted to look into how garbage collection time

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and other statistics,

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other parameters are evolving over time.

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And what I can see here is a cumulative distribution

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of GC duration for like first 10 minutes of Emacs uptime

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first 100 minutes first 1000 minutes.

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And if you look closer then you see

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that each individual garbage collection on average

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takes longer as you use Emacs longer.

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However this longer is not much it's like maybe 10 percent

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like basically garbage collection gets like

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slow Emacs down more as you use Emacs more but not much.

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So basically if you do you see Emacs

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being slower and slower over time

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it's probably not really garbage collection

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because it doesn't change too much.

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And if you look into time

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between individual garbage collections

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and you see that the time actually increases

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as you use Emacs longer which makes sense

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because initially like first few minutes

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you have all kind of packages loading

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like all the port loading and then later

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everything is loaded and things become more stable.

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So the conclusion on this part is that

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if Emacs becomes slower in a long session

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it's probably not caused by garbage collection.

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And one word of warning of course is that

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it's all nice and all when I present the statistics

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but it's only an average

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and if you are an actual user like here is one example

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which shows a total garbage collection time

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like accumulated together over Emacs uptime

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and you see different lines

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which correspond to different sessions of one user

00:16:45.560 --> 00:16:48.679
and you see they are wildly different

00:16:48.680 --> 00:16:51.439
like one time there is almost no garbage collection

00:16:51.440 --> 00:16:54.679
another time you see garbage collection

00:16:54.680 --> 00:16:56.999
because probably Emacs is used more early

00:16:57.000 --> 00:16:59.599
or like different pattern of usage

00:16:59.600 --> 00:17:03.159
and even during a single Emacs session

00:17:03.160 --> 00:17:04.599
you see a different slope

00:17:04.600 --> 00:17:06.439
of this curve which means that

00:17:06.440 --> 00:17:09.279
sometimes garbage collection is infrequent

00:17:09.280 --> 00:17:11.479
and sometimes it's much more frequent

00:17:11.480 --> 00:17:14.479
so it's probably much more noticeable one time

00:17:14.480 --> 00:17:15.639
and less noticeable other time.

00:17:15.640 --> 00:17:19.719
So if you think about these statistics of course

00:17:19.720 --> 00:17:23.359
they only represent an average usage

00:17:23.360 --> 00:17:26.359
but sometimes it can get worse sometimes it can get better.

00:17:26.360 --> 00:17:33.759
The last parameter I'd like to talk about is

00:17:33.760 --> 00:17:35.799
garbage collection during Emacs init.

00:17:35.800 --> 00:17:40.439
Basically if you think about what happens during Emacs init

00:17:40.440 --> 00:17:41.919
like when Emacs just starting up

00:17:41.920 --> 00:17:44.479
then whatever garbage collection

00:17:44.480 --> 00:17:46.759
there it's one or it's several times

00:17:46.760 --> 00:17:51.239
it all contributes to Emacs taking longer to start.

00:17:51.240 --> 00:17:56.559
And again we can look into the statistic

00:17:56.560 --> 00:18:01.159
and see what is the total GC duration after Emacs init

00:18:01.160 --> 00:18:06.159
and we see that 50% of all the submissions

00:18:06.160 --> 00:18:10.279
garbage collection adds up more than one second

00:18:10.280 --> 00:18:14.919
to Emacs init time and for 20% of users

00:18:14.920 --> 00:18:17.079
it's extra three seconds Emacs start time

00:18:17.080 --> 00:18:18.479
which is very significant

00:18:18.480 --> 00:18:21.479
especially for people who are used to Vim

00:18:21.480 --> 00:18:23.919
which can start in like a fraction of a second

00:18:23.920 --> 00:18:26.239
and here it just does garbage collection

00:18:26.240 --> 00:18:27.439
because garbage collection is not

00:18:27.440 --> 00:18:29.239
everything Emacs does during startup

00:18:29.240 --> 00:18:31.999
adds up more to the load.

00:18:32.000 --> 00:18:36.119
Okay that's all nice and all

00:18:36.120 --> 00:18:38.679
but what can we do about these statistics

00:18:38.680 --> 00:18:40.159
can we draw any conclusions

00:18:40.160 --> 00:18:43.239
and the answer is of course

00:18:43.240 --> 00:18:46.079
like the most important conclusion here is that

00:18:46.080 --> 00:18:49.439
yes garbage collection can slow down Emacs

00:18:49.440 --> 00:18:52.679
at least for some people and what to do about it

00:18:52.680 --> 00:18:55.319
there are two variables which you can tweak

00:18:55.320 --> 00:18:58.719
it's because gcconce threshold gcconce percentage

00:18:58.720 --> 00:19:03.159
and having the statistics I can at least look a little bit

00:19:03.160 --> 00:19:08.879
into what is the effect of increasing these variables

00:19:08.880 --> 00:19:12.439
like most people just increase gcconce threshold

00:19:12.440 --> 00:19:16.959
and like all the submissions people did increase

00:19:16.960 --> 00:19:19.919
and doesn't make much sense to decrease it

00:19:19.920 --> 00:19:21.079
like to make things worse

00:19:21.080 --> 00:19:27.639
of course for these statistics

00:19:27.640 --> 00:19:31.559
the exact values of this increased thresholds

00:19:31.560 --> 00:19:33.839
are not always the same

00:19:33.840 --> 00:19:36.479
but at least we can look into some trends

00:19:36.480 --> 00:19:44.759
so first and obvious thing we can observe

00:19:44.760 --> 00:19:46.759
is when we compare

00:19:46.760 --> 00:19:50.399
the standard gc settings standard thresholds

00:19:50.400 --> 00:19:53.999
and increased thresholds for time between

00:19:54.000 --> 00:19:57.479
subsequent gcs and as one may expect

00:19:57.480 --> 00:19:59.559
if you increase the threshold

00:19:59.560 --> 00:20:02.679
Emacs will do garbage collection less frequently

00:20:02.680 --> 00:20:05.279
so the spacing between garbage collection increases

00:20:05.280 --> 00:20:07.599
okay the only thing is that

00:20:07.600 --> 00:20:10.719
if garbage collection is less frequent

00:20:10.720 --> 00:20:14.079
then each individual garbage collection becomes longer

00:20:14.080 --> 00:20:18.159
so if you think about increasing

00:20:18.160 --> 00:20:22.239
garbage collection thresholds be prepared

00:20:22.240 --> 00:20:26.519
that in each individual time Emacs freezes will take longer

00:20:26.520 --> 00:20:31.599
this is one caveat when we talk about

00:20:31.600 --> 00:20:34.079
this agglomerated gcs which are one after other

00:20:34.080 --> 00:20:36.759
like if you increase the threshold sufficiently

00:20:36.760 --> 00:20:42.319
then whatever happened that garbage collections

00:20:42.320 --> 00:20:44.399
were like done one after other

00:20:44.400 --> 00:20:47.599
we can now make it so that they are actually separated

00:20:47.600 --> 00:20:51.559
so like you don't see one giant freeze caused by

00:20:51.560 --> 00:20:52.919
like 10 gcs in a row

00:20:52.920 --> 00:20:55.759
instead you can make it so that they are separated

00:20:55.760 --> 00:20:59.079
and in statistics it's very clear

00:20:59.080 --> 00:21:02.959
that the number of agglomerated garbage collections

00:21:02.960 --> 00:21:06.919
decreases dramatically when you increase the thresholds

00:21:06.920 --> 00:21:11.759
it's particularly evident when we look into startup time

00:21:11.760 --> 00:21:17.279
if you look at gc duration during Emacs startup

00:21:17.280 --> 00:21:19.159
and if we look into what happens

00:21:19.160 --> 00:21:20.879
when you increase the thresholds

00:21:20.880 --> 00:21:23.799
it's very clear that Emacs startup become faster

00:21:23.800 --> 00:21:26.279
when you increase gc thresholds

00:21:26.280 --> 00:21:33.359
so that's all for actual user statistics

00:21:33.360 --> 00:21:35.439
and now let's try to run into

00:21:35.440 --> 00:21:38.079
some like actual recommendations

00:21:38.080 --> 00:21:42.639
on what numbers to set and before we start

00:21:42.640 --> 00:21:44.399
let me explain a little bit about

00:21:44.400 --> 00:21:46.479
the difference between these two variables

00:21:46.480 --> 00:21:48.879
which is gc constant threshold and gc constant percentage

00:21:48.880 --> 00:21:52.359
so if you think about Emacs memory

00:21:52.360 --> 00:21:55.239
like there's a certain memory allocated by Emacs

00:21:55.240 --> 00:21:58.479
and then as you run commands and turn using Emacs

00:21:58.480 --> 00:22:00.079
there is more memory allocated

00:22:00.080 --> 00:22:04.639
and Emacs decides when to do garbage collection

00:22:04.640 --> 00:22:06.079
according these two variables

00:22:06.080 --> 00:22:08.759
and actually what it does it chooses the larger one

00:22:08.760 --> 00:22:12.119
so say you have you are late in Emacs session

00:22:12.120 --> 00:22:14.039
you have a lot of Emacs memory allocated

00:22:14.040 --> 00:22:17.119
then you have gc constant percentage

00:22:17.120 --> 00:22:19.919
which is percent of the already allocated memory

00:22:19.920 --> 00:22:25.119
and that percent is probably going to be the largest

00:22:25.120 --> 00:22:28.319
because you have more memory

00:22:28.320 --> 00:22:32.559
and memory means that percent of it is larger

00:22:32.560 --> 00:22:36.359
so like you have a larger number cost

00:22:36.360 --> 00:22:37.719
by gc constant percentage

00:22:37.720 --> 00:22:43.079
so in this scenario when Emacs session is already running

00:22:43.080 --> 00:22:45.319
for a long time and there is a lot of memory allocated

00:22:45.320 --> 00:22:50.119
you have gc constant percentage

00:22:50.120 --> 00:22:52.279
controlling the garbage collection

00:22:52.280 --> 00:22:54.999
while early in Emacs there is not much memory placed

00:22:55.000 --> 00:22:58.719
Emacs just starting up then gc constant threshold

00:22:58.720 --> 00:23:01.639
is controlling how frequently garbage collection happens

00:23:01.640 --> 00:23:04.799
because smaller allocated memory

00:23:04.800 --> 00:23:06.839
means its percentage will be a small number

00:23:06.840 --> 00:23:12.319
so in terms of default values at least

00:23:12.320 --> 00:23:14.239
gc constant threshold is 800 kilobytes

00:23:14.240 --> 00:23:18.799
and gc constant percentage is 10

00:23:18.800 --> 00:23:24.159
so gc constant percentage becomes larger than that threshold

00:23:24.160 --> 00:23:28.919
when you have more than eight megabytes of allocated memory

00:23:28.920 --> 00:23:31.039
by Emacs which is quite early

00:23:31.040 --> 00:23:34.279
and it will probably hold just during the startup

00:23:34.280 --> 00:23:36.799
and once you start using your maximum

00:23:36.800 --> 00:23:38.919
and once you load all the histories

00:23:38.920 --> 00:23:42.039
all the kinds of buffers it's probably going to take

00:23:42.040 --> 00:23:43.959
more than much more than eight megabytes

00:23:43.960 --> 00:23:50.639
so now we understand this

00:23:50.640 --> 00:23:53.279
we can draw certain recommendations

00:23:53.280 --> 00:23:57.279
about tweaking the gc thresholds

00:23:57.280 --> 00:24:01.159
so first of all I need to emphasize

00:24:01.160 --> 00:24:03.639
that any time you increase gc threshold

00:24:03.640 --> 00:24:07.199
an individual garbage collection time increases

00:24:07.200 --> 00:24:08.759
so it's not free at all

00:24:08.760 --> 00:24:10.999
if you don't have problems with garbage collection

00:24:11.000 --> 00:24:13.519
which is half of the users don't have much problem

00:24:13.520 --> 00:24:15.079
you don't need to tweak anything

00:24:15.080 --> 00:24:19.359
only when gc is frequent and slow

00:24:19.360 --> 00:24:23.399
when Emacs is really really present frequently

00:24:23.400 --> 00:24:27.119
you may consider increasing gc thresholds only

00:24:27.120 --> 00:24:31.479
and in particular I recommend

00:24:31.480 --> 00:24:33.279
increasing gc constant percentage

00:24:33.280 --> 00:24:36.359
because that's what mostly controls gc

00:24:36.360 --> 00:24:40.079
when Emacs is running for long session

00:24:40.080 --> 00:24:43.039
and the numbers are probably like

00:24:43.040 --> 00:24:46.519
yeah we can estimate the effect of these numbers

00:24:46.520 --> 00:24:49.679
like for example if you have a default value of 0.1 percent

00:24:49.680 --> 00:24:52.759
for gc constant percentage 0.1 which is 10 percent

00:24:52.760 --> 00:24:55.039
and then increase it twice

00:24:55.040 --> 00:24:58.639
obviously you get twice less frequent gcs

00:24:58.640 --> 00:25:02.559
but it will come at the cost of extra 10 percent gc time

00:25:02.560 --> 00:25:05.839
and if you increase 10 times you can think about

00:25:05.840 --> 00:25:08.719
10 less 10 x less frequent gcs

00:25:08.720 --> 00:25:12.199
but almost twice longer individual garbage collection time

00:25:12.200 --> 00:25:16.919
so probably you want to set the number closer to 0.1

00:25:16.920 --> 00:25:23.399
another part of the users may actually

00:25:23.400 --> 00:25:28.359
try to optimize Emacs startup time

00:25:28.360 --> 00:25:30.759
which is quite frequent problem

00:25:30.760 --> 00:25:34.919
in this case it's probably better to increase gc constant

00:25:34.920 --> 00:25:38.199
but not too much so like

00:25:38.200 --> 00:25:40.239
first of all it makes sense to check

00:25:40.240 --> 00:25:43.319
whether garbage collection is a problem at all

00:25:43.320 --> 00:25:45.999
during startup and there are two variables

00:25:46.000 --> 00:25:50.199
which can show what is happening this garbage collection

00:25:50.200 --> 00:25:53.719
so gc done is a variable that shows

00:25:53.720 --> 00:25:55.039
how many garbage collection

00:25:55.040 --> 00:26:00.159
like what is the number of garbage collections triggered

00:26:00.160 --> 00:26:02.599
like when you check the value

00:26:02.600 --> 00:26:04.039
or right after you start Emacs

00:26:04.040 --> 00:26:04.799
you will see that

00:26:04.800 --> 00:26:08.519
number and gc elapsed variable

00:26:08.520 --> 00:26:11.599
which gives you a number of seconds

00:26:11.600 --> 00:26:14.959
which Emacs spent in doing garbage collection

00:26:14.960 --> 00:26:16.879
so this is probably the most important variable

00:26:16.880 --> 00:26:20.719
and if you see it's large then you may consider tweaking it

00:26:20.720 --> 00:26:26.799
for the Emacs startup we can estimate some bounds

00:26:26.800 --> 00:26:30.039
because in the statistics I never saw anything

00:26:30.040 --> 00:26:32.439
that is more than 10 seconds extra

00:26:32.440 --> 00:26:34.439
which even 10 seconds is probably like

00:26:34.440 --> 00:26:39.119
a really really hard upper bound so

00:26:39.120 --> 00:26:44.479
or say if you want to decrease the gc contribution

00:26:44.480 --> 00:26:47.479
like order of magnitude or like two orders of magnitudes

00:26:47.480 --> 00:26:50.879
let's say like as a really hard top estimate

00:26:50.880 --> 00:26:55.079
then it corresponds to 80 megabytes gc constant

00:26:55.080 --> 00:26:58.959
and probably much less so like

00:26:58.960 --> 00:27:00.679
there's no point setting it

00:27:00.680 --> 00:27:04.159
to a few hundred megabytes of course

00:27:04.160 --> 00:27:08.439
there's one caveat which is important to keep in

00:27:08.440 --> 00:27:14.039
mind though that increasing the gc thresholds

00:27:14.040 --> 00:27:16.399
is not just increasing individual gc time

00:27:16.400 --> 00:27:20.399
there's also an actual real impact on the RAM usage

00:27:20.400 --> 00:27:23.839
so like if you increase gc threshold

00:27:23.840 --> 00:27:26.879
it increases the RAM usage of Emacs

00:27:26.880 --> 00:27:29.639
and you shouldn't think that like okay

00:27:29.640 --> 00:27:33.159
I increased the threshold by like 100 megabytes

00:27:33.160 --> 00:27:37.119
then 100 megabytes extra RAM usage doesn't matter

00:27:37.120 --> 00:27:38.679
it's not 100 megabytes

00:27:38.680 --> 00:27:42.319
because less frequent garbage collection means

00:27:42.320 --> 00:27:45.639
it will lead to memory fragmentation

00:27:45.640 --> 00:27:50.439
so in practice if you increase the thresholds

00:27:50.440 --> 00:27:52.799
to tens or hundreds of megabytes

00:27:52.800 --> 00:27:55.919
we are talking about gigabytes extra RAM usage

00:27:55.920 --> 00:27:59.719
for me personally when I tried to play with gc thresholds

00:27:59.720 --> 00:28:02.879
I have seen Emacs taking two gigabytes like

00:28:02.880 --> 00:28:05.519
compared to several times less

00:28:05.520 --> 00:28:09.039
when with default settings so it's not free at all

00:28:09.040 --> 00:28:13.639
and only like either when you have a lot of free RAM

00:28:13.640 --> 00:28:16.839
and you don't care or when your Emacs is really slow

00:28:16.840 --> 00:28:19.559
then you may need to consider this

00:28:19.560 --> 00:28:23.239
tweaking these defaults so again don't tweak defaults

00:28:23.240 --> 00:28:24.239
if you don't really have a problem

00:28:24.240 --> 00:28:29.839
and of course this RAM problem is a big big deal

00:28:29.840 --> 00:28:35.679
for Emacs devs because from from the point of single user

00:28:35.680 --> 00:28:38.839
you have like normal laptop most likely like normal PC

00:28:38.840 --> 00:28:42.079
with a lot of RAM you don't care about these things too much

00:28:42.080 --> 00:28:48.999
but Emacs in general can run on like all kinds of machines

00:28:49.000 --> 00:28:51.679
including low-end machines with very limited RAM

00:28:51.680 --> 00:28:55.359
and anytime Emacs developers consider increasing

00:28:55.360 --> 00:28:57.959
the defaults for garbage collection

00:28:57.960 --> 00:29:01.479
it's like they always have to consider

00:29:01.480 --> 00:29:02.959
if you increase them too much

00:29:02.960 --> 00:29:07.919
then Emacs may just stop running on certain platforms

00:29:07.920 --> 00:29:14.439
so that's a very big consideration in terms

00:29:14.440 --> 00:29:16.639
of the global defaults for everyone

00:29:16.640 --> 00:29:22.199
although I have to I would say that it might be related

00:29:22.200 --> 00:29:24.479
to the safe to increase GCCons threshold

00:29:24.480 --> 00:29:27.919
because it mostly affects startup and during startup

00:29:27.920 --> 00:29:31.279
it's probably not the peak usage of Emacs

00:29:31.280 --> 00:29:35.599
and like as Emacs runs for longer

00:29:35.600 --> 00:29:38.199
it's probably where most of RAM will be used later

00:29:38.200 --> 00:29:44.399
on the other hand GCCons percentage is much more debating

00:29:44.400 --> 00:29:46.159
because it has pros and cons

00:29:46.160 --> 00:29:47.719
it will increase the RAM usage

00:29:47.720 --> 00:29:50.999
it will increase the individual GC time so

00:29:51.000 --> 00:29:56.119
if we consider changing it it's much more tricky

00:29:56.120 --> 00:29:59.479
and we have discussing probably measure the impact on users

00:29:59.480 --> 00:30:05.799
and a final note on or from the point of view

00:30:05.800 --> 00:30:07.319
of Emacs development is

00:30:07.320 --> 00:30:11.039
that this simple mark-and-sweep algorithm

00:30:11.040 --> 00:30:14.119
is like a very old and not the state-of-the-art algorithm

00:30:14.120 --> 00:30:17.799
there are variants of garbage collection

00:30:17.800 --> 00:30:19.479
that are like totally non-blocking

00:30:19.480 --> 00:30:22.479
so Emacs just doesn't have to freeze

00:30:22.480 --> 00:30:24.279
during the garbage collection

00:30:24.280 --> 00:30:26.839
or there are variants of garbage collection algorithm

00:30:26.840 --> 00:30:30.079
that do not scan all the memory just fraction of it

00:30:30.080 --> 00:30:33.439
and scan another fraction less frequently

00:30:33.440 --> 00:30:36.999
so there are actually ways just to change

00:30:37.000 --> 00:30:39.799
the garbage collection algorithm to make things much faster

00:30:39.800 --> 00:30:44.199
of course like just changing the numbers of variables

00:30:44.200 --> 00:30:47.079
like the numbers of variable values

00:30:47.080 --> 00:30:50.079
is much more tricky and one has to implement it

00:30:50.080 --> 00:30:52.239
obviously it would be nice if someone implements it

00:30:52.240 --> 00:30:55.639
but so far it's not happening so yeah it would be nice

00:30:55.640 --> 00:30:59.359
but maybe not not so quickly

00:30:59.360 --> 00:31:02.159
there is more chance to change the defaults here

00:31:02.160 --> 00:31:07.479
to conclude let me reiterate the most important points

00:31:07.480 --> 00:31:11.919
so from point of view of users you need to understand that

00:31:11.920 --> 00:31:14.479
yes garbage collection may be a problem

00:31:14.480 --> 00:31:16.679
but not for everyone so like

00:31:16.680 --> 00:31:21.079
you should only think about changing the variables

00:31:21.080 --> 00:31:23.559
when you really know that garbage collection

00:31:23.560 --> 00:31:27.479
is the problem for you so if you have slow Emacs startup

00:31:27.480 --> 00:31:30.919
slow Emacs startup and you know that it's caused by

00:31:30.920 --> 00:31:32.479
garbage collection like by

00:31:32.480 --> 00:31:35.999
you can check the GC elapsed variable

00:31:36.000 --> 00:31:39.679
then you may increase GC count threshold

00:31:39.680 --> 00:31:42.119
like to few tens of megabytes not more

00:31:42.120 --> 00:31:44.479
it doesn't make sense to increase it much more

00:31:44.480 --> 00:31:48.239
and if you really have major problems

00:31:48.240 --> 00:31:49.759
with Emacs being slaggy

00:31:49.760 --> 00:31:52.519
then you can increase GC count percentage

00:31:52.520 --> 00:31:55.999
to like 0.2 0.3 maybe

00:31:56.000 --> 00:31:57.679
one is probably overkill

00:31:57.680 --> 00:32:02.759
but do watch your Emacs ROM usage it may be really impacted

00:32:02.760 --> 00:32:09.719
for Emacs developers I'd like to emphasize

00:32:09.720 --> 00:32:12.439
that there is a real problem with garbage collection

00:32:12.440 --> 00:32:17.959
and nine percent of all the garbage collection

00:32:17.960 --> 00:32:22.079
data points we have correspond

00:32:22.080 --> 00:32:24.959
to really slow noticeable Emacs precision

00:32:24.960 --> 00:32:28.039
and really frequent less than 10 seconds

00:32:28.040 --> 00:32:32.319
I'd say that it's really worth

00:32:32.320 --> 00:32:35.279
increasing GC count threshold at least during startup

00:32:35.280 --> 00:32:40.159
because it really impacts the Emacs startup time

00:32:40.160 --> 00:32:41.519
making Emacs startup much faster

00:32:41.520 --> 00:32:44.799
ideally we need to reimplement

00:32:44.800 --> 00:32:48.599
the garbage collection algorithm of course it's not easy

00:32:48.600 --> 00:32:50.159
but it would be really nice

00:32:50.160 --> 00:32:56.399
and for GC count percentage defaults it's hard to say

00:32:56.400 --> 00:33:00.759
we may consider changing it but it's up to discussion

00:33:00.760 --> 00:33:03.119
and we probably need to be conservative here

00:33:03.120 --> 00:33:06.039
so we came to the end of my talk

00:33:06.040 --> 00:33:09.319
and this presentation

00:33:09.320 --> 00:33:11.839
all the data will be available publicly

00:33:11.840 --> 00:33:17.079
and you can reproduce all the statistic graphs if you wish

00:33:17.080 --> 00:33:21.920
and thank you for attention