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p5314: add a size test for name-hash collisions
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Add a new test-tool helper, name-hash, to output the value of the
name-hash algorithms for the input list of strings, one per line.

Create a performance test that uses test_size to demonstrate how
collisions occur for these hash algorithms. This test helps inform
someone as to the behavior of the name-hash algorithms for their repo
based on the paths at HEAD.

My copy of the Git repository shows modest statistics around the
collisions of the default name-hash algorithm:

Test                                              this tree
-----------------------------------------------------------------
5314.1: paths at head                                        4.5K
5314.2: number of distinct name-hashes                       4.1K
5314.3: number of distinct full-name-hashes                  4.5K
5314.4: maximum multiplicity of name-hashes                    13
5314.5: maximum multiplicity of fullname-hashes                 1

Here, the maximum collision multiplicity is 13, but around 10% of paths
have a collision with another path.

In a more interesting example, the microsoft/fluentui [1] repo had these
statistics at time of committing:

Test                                              this tree
-----------------------------------------------------------------
5314.1: paths at head                                       19.6K
5314.2: number of distinct name-hashes                       8.2K
5314.3: number of distinct full-name-hashes                 19.6K
5314.4: maximum multiplicity of name-hashes                   279
5314.5: maximum multiplicity of fullname-hashes                 1

[1] https://github.com/microsoft/fluentui

That demonstrates that of the nearly twenty thousand path names, they
are assigned around eight thousand distinct values. 279 paths are
assigned to a single value, leading the packing algorithm to sort
objects from those paths together, by size.

In this repository, no collisions occur for the full-name-hash
algorithm.

In a more extreme example, an internal monorepo had a much worse
collision rate:

Test                                              this tree
-----------------------------------------------------------------
5314.1: paths at head                                      221.6K
5314.2: number of distinct name-hashes                      72.0K
5314.3: number of distinct full-name-hashes                221.6K
5314.4: maximum multiplicity of name-hashes                 14.4K
5314.5: maximum multiplicity of fullname-hashes                 2

Even in this repository with many more paths at HEAD, the collision rate
was low and the maximum number of paths being grouped into a single
bucket by the full-path-name algorithm was two.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
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derrickstolee committed Sep 11, 2024
1 parent 6309e2b commit 0ec4b66
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1 change: 1 addition & 0 deletions Makefile
Original file line number Diff line number Diff line change
Expand Up @@ -812,6 +812,7 @@ TEST_BUILTINS_OBJS += test-lazy-init-name-hash.o
TEST_BUILTINS_OBJS += test-match-trees.o
TEST_BUILTINS_OBJS += test-mergesort.o
TEST_BUILTINS_OBJS += test-mktemp.o
TEST_BUILTINS_OBJS += test-name-hash.o
TEST_BUILTINS_OBJS += test-oid-array.o
TEST_BUILTINS_OBJS += test-online-cpus.o
TEST_BUILTINS_OBJS += test-pack-mtimes.o
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23 changes: 23 additions & 0 deletions t/helper/test-name-hash.c
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@@ -0,0 +1,23 @@
/*
* test-name-hash.c: Read a list of paths over stdin and report on their
* name-hash and full name-hash.
*/

#include "test-tool.h"
#include "git-compat-util.h"
#include "pack-objects.h"
#include "strbuf.h"

int cmd__name_hash(int argc UNUSED, const char **argv UNUSED)
{
struct strbuf line = STRBUF_INIT;

while (!strbuf_getline(&line, stdin)) {
uint32_t name_hash = pack_name_hash(line.buf);
uint32_t full_hash = pack_full_name_hash(line.buf);

printf("%10"PRIu32"\t%10"PRIu32"\t%s\n", name_hash, full_hash, line.buf);
}

return 0;
}
1 change: 1 addition & 0 deletions t/helper/test-tool.c
Original file line number Diff line number Diff line change
Expand Up @@ -43,6 +43,7 @@ static struct test_cmd cmds[] = {
{ "match-trees", cmd__match_trees },
{ "mergesort", cmd__mergesort },
{ "mktemp", cmd__mktemp },
{ "name-hash", cmd__name_hash },
{ "oid-array", cmd__oid_array },
{ "online-cpus", cmd__online_cpus },
{ "pack-mtimes", cmd__pack_mtimes },
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1 change: 1 addition & 0 deletions t/helper/test-tool.h
Original file line number Diff line number Diff line change
Expand Up @@ -37,6 +37,7 @@ int cmd__lazy_init_name_hash(int argc, const char **argv);
int cmd__match_trees(int argc, const char **argv);
int cmd__mergesort(int argc, const char **argv);
int cmd__mktemp(int argc, const char **argv);
int cmd__name_hash(int argc, const char **argv);
int cmd__online_cpus(int argc, const char **argv);
int cmd__pack_mtimes(int argc, const char **argv);
int cmd__parse_options(int argc, const char **argv);
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41 changes: 41 additions & 0 deletions t/perf/p5314-name-hash.sh
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@@ -0,0 +1,41 @@
#!/bin/sh

test_description='Tests pack performance using bitmaps'
. ./perf-lib.sh

GIT_TEST_PASSING_SANITIZE_LEAK=0
export GIT_TEST_PASSING_SANITIZE_LEAK

test_perf_large_repo

test_size 'paths at head' '
git ls-tree -r --name-only HEAD >path-list &&
wc -l <path-list
'

test_size 'number of distinct name-hashes' '
cat path-list | test-tool name-hash >name-hashes &&
cat name-hashes | awk "{ print \$1; }" | sort -n | uniq -c >name-hash-count &&
wc -l <name-hash-count
'

test_size 'number of distinct full-name-hashes' '
cat name-hashes | awk "{ print \$2; }" | sort -n | uniq -c >full-name-hash-count &&
wc -l <full-name-hash-count
'

test_size 'maximum multiplicity of name-hashes' '
cat name-hash-count | \
sort -nr | \
head -n 1 | \
awk "{ print \$1; }"
'

test_size 'maximum multiplicity of fullname-hashes' '
cat full-name-hash-count | \
sort -nr | \
head -n 1 | \
awk "{ print \$1; }"
'

test_done

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