Re: [PATCH/RFC] gitperformance: add new documentation about git performance tuning

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Tue, Apr 4, 2017 at 5:07 PM, Jeff Hostetler <git@jeffhostetler.com> wrote:
>
> On 4/3/2017 5:16 PM, Ævar Arnfjörð Bjarmason wrote:
>>
>> Add a new manpage that gives an overview of how to tweak git's
>> performance.
>>
>> There's currently no good single resource for things a git site
>> administrator might want to look into to improve performance for his
>> site & his users. This unfinished documentation aims to be the first
>> thing someone might want to look at when investigating ways to improve
>> git performance.
>>
>> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
>> ---
>>
>> I've been wanting to get something like this started for a while. It's
>> obviously woefully incomplete. Pointers about what to include would be
>> great & whether including something like this makes sense.
>>
>> Things I have on my TODO list:
>>
>>  - Add a section discussing how refs impact performance, suggest
>>    e.g. archiving old tags if possible, or at least run "git remote
>>    prune origin" regularly on clients.
>>
>>  - Discuss split index a bit, although I'm not very confident in
>>    describing what its pros & cons are.
>>
>>  - Should we be covering good practices for your repo going forward to
>>    maintain good performance? E.g. don't have some huge tree all in
>>    one directory (use subdirs), don't add binary (rather
>>    un-delta-able) content if you can help it etc.
>>
>> - The new core.checksumIndex option being discussed on-list. Which
>>   actually drove my to finally write this up (hrm, this sounds useful,
>>   but unless I was watching the list I'd probably never see it...).
>
>
> You might also consider core.preloadIndex.

It's been enabled by default since 2.1.0 (299e29870b), or do you mean
talk about disabling it, or "this is a perf option we have on by
default"?

I don't know the pros of disabling that, haven't used it myself & it's
not clear from the docs.

> For people with very large trees, talk about sparse-checkout.

*nod*

> And (on Windows) core.fscache.  Or leave a place for
> an addendum for Windows that we can fill in later.

I have no core.fscache in my git.git, did you mean something else?

>
>
>>
>>
>>  Documentation/Makefile           |   1 +
>>  Documentation/gitperformance.txt | 107
>> +++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 108 insertions(+)
>>  create mode 100644 Documentation/gitperformance.txt
>>
>> diff --git a/Documentation/Makefile b/Documentation/Makefile
>> index b5be2e2d3f..528aa22354 100644
>> --- a/Documentation/Makefile
>> +++ b/Documentation/Makefile
>> @@ -23,6 +23,7 @@ MAN5_TXT += gitrepository-layout.txt
>>  MAN5_TXT += gitweb.conf.txt
>>
>>  MAN7_TXT += gitcli.txt
>> +MAN7_TXT += gitperformance.txt
>>  MAN7_TXT += gitcore-tutorial.txt
>>  MAN7_TXT += gitcredentials.txt
>>  MAN7_TXT += gitcvs-migration.txt
>> diff --git a/Documentation/gitperformance.txt
>> b/Documentation/gitperformance.txt
>> new file mode 100644
>> index 0000000000..0548d1e721
>> --- /dev/null
>> +++ b/Documentation/gitperformance.txt
>> @@ -0,0 +1,107 @@
>> +giteveryday(7)
>> +==============
>> +
>> +NAME
>> +----
>> +gitperformance - How to improve Git's performance
>> +
>> +SYNOPSIS
>> +--------
>> +
>> +A guide to improving Git's performance beyond the defaults.
>> +
>> +DESCRIPTION
>> +-----------
>> +
>> +Git is mostly performant by default, but ships with various
>> +configuration options, command-line options, etc. that might improve
>> +performance, but for various reasons aren't on by default.
>> +
>> +This document provides a brief overview of these features.
>> +
>> +The reader should not assume that turning on all of these features
>> +will increase performance, depending on the repository, workload &
>> +use-case turning some of them on might severely harm performance.
>> +
>> +This document serves as a starting point for things to look into when
>> +it comes to improving performance, not as a checklist for things to
>> +enable or disable.
>> +
>> +Performance by topic
>> +--------------------
>> +
>> +It can be hard to divide the performance features into topics, but
>> +most of them fall into various well-defined buckets. E.g. there are
>> +features that help with the performance of "git status", and couldn't
>> +possibly impact repositories without working copies, and then some
>> +that only impact the performance of cloning from a server, or help the
>> +server itself etc.
>> +
>> +git status
>> +~~~~~~~~~~
>> +
>> +Running "git status" requires traversing the working tree & comparing
>> +it with the index. Several configuration options can help with its
>> +performance, with some trade-offs.
>> +
>> +- config: "core.untrackedCache=true" (see linkgit:git-config[1]) can
>> +  save on `stat(2)` calls by caching the mtime of filesystem
>> +  directories, and if they didn't change avoid recursing into that
>> +  directory to `stat(2)` every file in them.
>> ++
>> +pros: Can drastically speed up "git status".
>> ++
>> +cons: There's a speed hit for initially populating & maintaining the
>> +cache. Doesn't work on all filesystems (see `--test-untracked-cache`
>> +in linkgit:git-update-index[1]).
>> +
>> +- config: "status.showUntrackedFiles=no" (see
>> +  linkgit:git-config[1]). Skips looking for files in the working tree
>> +  git doesn't already know about.
>> ++
>> +pros: Speeds up "git status" by making it do a lot less work.
>> ++
>> +cons: If there's any new & untracked files anywhere in the working
>> +tree they won't be noticed by git. Makes it easy to accidentally miss
>> +files to "git add" before committing, or files which might impact the
>> +code in the working tree, but which git won't know exist.
>> +
>> +git grep
>> +~~~~~~~~
>> +
>> +- config: "grep.patternType=perl" (see linkgit:git-config[1]) will use
>> +  the PCRE library when "git grep" is invoked by default. This can be
>> +  faster than POSIX regular expressions in many cases.
>> ++
>> +pros: Can, depending on the use-case, be faster than default "git grep".
>> ++
>> +cons: Can also be slower, and in some edge cases produce different
>> +results.
>> +
>> +- config: "grep.threads=*" (see linkgit:git-config[1] &
>> +  linkgit:git-grep[1]). Tunes the number of "git grep" worker threads.
>> ++
>> +pros: Giving this a more optimal value might result in a faster grep.
>> ++
>> +cons: It might not.
>> +
>> +Server options to help clients
>> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> +
>> +These features can be enabled on git servers, they won't help the
>> +performance of the servers themselves, but will help clients that need
>> +to talk to those servers.
>> +
>> +- config: "repack.writeBitmaps=true" (see
>> +  linkgit:git-config[1]). Spend more time during repack to produce
>> +  bitmap index, helps clients with "fetch" & "clone" performance.
>> ++
>> +pros: Once enabled & run regularly as part of "git repack" speeds up
>> +"clone" and "fetch".
>> ++
>> +cons: Takes extra time during repack, requires doing full
>> +non-incremental repacks with `-A` or `-a`.
>> +
>> +GIT
>> +---
>> +Part of the linkgit:git[1] suite
>>
>