Git performance tests

This directory holds performance testing scripts for git tools. The first part of this document describes the various ways in which you can run them.

When fixing the tools or adding enhancements, you are strongly encouraged to add tests in this directory to cover what you are trying to fix or enhance. The later part of this short document describes how your test scripts should be organized.

Running Tests

The easiest way to run tests is to say "make". This runs all the tests on the current git repository.

=== Running 2 tests in this tree ===
[...]
Test                                     this tree
---------------------------------------------------------
0001.1: rev-list --all                   0.54(0.51+0.02)
0001.2: rev-list --all --objects         6.14(5.99+0.11)
7810.1: grep worktree, cheap regex       0.16(0.16+0.35)
7810.2: grep worktree, expensive regex   7.90(29.75+0.37)
7810.3: grep --cached, cheap regex       3.07(3.02+0.25)
7810.4: grep --cached, expensive regex   9.39(30.57+0.24)

Output format is in seconds "Elapsed(User + System)"

You can compare multiple repositories and even git revisions with the 'run' script:

$ ./run . origin/next /path/to/git-tree p0001-rev-list.sh

where . stands for the current git tree. The full invocation is

./run [<revision|directory>...] [--] [<test-script>...]

A '.' argument is implied if you do not pass any other revisions/directories.

You can also manually test this or another git build tree, and then call the aggregation script to summarize the results:

$ ./p0001-rev-list.sh
[...]
$ ./run /path/to/other/git -- ./p0001-rev-list.sh
[...]
$ ./aggregate.perl . /path/to/other/git ./p0001-rev-list.sh

aggregate.perl has the same invocation as 'run', it just does not run anything beforehand.

You can set the following variables (also in your config.mak):

GIT_PERF_REPEAT_COUNT
Number of times a test should be repeated for best-of-N
measurements.  Defaults to 3.

GIT_PERF_MAKE_OPTS
Options to use when automatically building a git tree for
performance testing. E.g., -j6 would be useful. Passed
directly to make as "make $GIT_PERF_MAKE_OPTS".

GIT_PERF_MAKE_COMMAND
An arbitrary command that'll be run in place of the make
command, if set the GIT_PERF_MAKE_OPTS variable is
ignored. Useful in cases where source tree changes might
require issuing a different make command to different
revisions.

This can be (ab)used to monkeypatch or otherwise change the
tree about to be built. Note that the build directory can be
re-used for subsequent runs so the make command might get
executed multiple times on the same tree, but don't count on
any of that, that's an implementation detail that might change
in the future.

GIT_PERF_REPO
GIT_PERF_LARGE_REPO
Repositories to copy for the performance tests.  The normal
repo should be at least git.git size.  The large repo should
probably be about linux.git size for optimal results.
Both default to the git.git you are running from.

GIT_PERF_EXTRA
Boolean to enable additional tests. Most test scripts are
written to detect regressions between two versions of Git, and
the output will compare timings for individual tests between
those versions. Some scripts have additional tests which are not
run by default, that show patterns within a single version of
Git (e.g., performance of index-pack as the number of threads
changes). These can be enabled with GIT_PERF_EXTRA.

GIT_PERF_USE_SCALAR
Boolean indicating whether to register test repo(s) with Scalar
before executing tests.

You can also pass the options taken by ordinary git tests; the most useful one is:

--root=:: Create "trash" directories used to store all temporary data during testing under , instead of the t/ directory. Using this option with a RAM-based filesystem (such as tmpfs) can massively speed up the test suite.

Naming Tests

The performance test files are named as:

pNNNN-commandname-details.sh

where N is a decimal digit. The same conventions for choosing NNNN as for normal tests apply.

Writing Tests

The perf script starts much like a normal test script, except it sources perf-lib.sh:

#!/bin/sh
#
# Copyright (c) 2005 Junio C Hamano
#

test_description='xxx performance test'
. ./perf-lib.sh

After that you will want to use some of the following:

test_perf_fresh_repo    # sets up an empty repository
test_perf_default_repo  # sets up a "normal" repository
test_perf_large_repo    # sets up a "large" repository

test_perf_default_repo sub  # ditto, in a subdir "sub"

    test_checkout_worktree  # if you need the worktree too

At least one of the first two is required!

You can use test_expect_success as usual. In both test_expect_success and in test_perf, running "git" points to the version that is being perf-tested. The $MODERN_GIT variable points to the git wrapper for the currently checked-out version (i.e., the one that matches the t/perf scripts you are running). This is useful if your setup uses commands that only work with newer versions of git than what you might want to test (but obviously your new commands must still create a state that can be used by the older version of git you are testing).

For actual performance tests, use

test_perf 'descriptive string' '
	command1 &&
	command2
'

test_perf spawns a subshell, for lack of better options. This means that

• you must export all variables that you need in the subshell

• you must flag all variables that you want to persist from the subshell with 'test_export':

  test_perf 'descriptive string' ' foo=$(git rev-parse HEAD) && test_export foo '

  The so-exported variables are automatically marked for export in the shell executing the perf test. For your convenience, test_export is the same as export in the main shell.

  This feature relies on a bit of magic using 'set' and 'source'. While we have tried to make sure that it can cope with embedded whitespace and other special characters, it will not work with multi-line data.

Rather than tracking the performance by run-time as test_perf does, you may also track output size by using test_size. The stdout of the function should be a single numeric value, which will be captured and shown in the aggregated output. For example:

test_perf 'time foo' '
	./foo >foo.out
'

test_size 'output size'
	wc -c <foo.out
'

might produce output like:

Test                origin           HEAD
-------------------------------------------------------------
1234.1 time foo     0.37(0.79+0.02)  0.26(0.51+0.02) -29.7%
1234.2 output size             4.3M             3.6M -14.7%

The item being measured (and its units) is up to the test; the context and the test title should make it clear to the user whether bigger or smaller numbers are better. Unlike test_perf, the test code will only be run once, since output sizes tend to be more deterministic than timings.