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
00:14:58.560 --> 00:15:03.719
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.
00:15:34.720 --> 00:15:38.359
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
00:16:01.480 --> 00:16:04.239
like all the port loading and then later
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everything is loaded and things become more stable.
00:16:07.240 --> 00:16:12.879
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
