From: Derrick Stolee Date: Mon, 27 Jan 2025 19:02:33 +0000 (+0000) Subject: test-tool: add helper for name-hash values X-Git-Tag: v2.49.0-rc0~43^2~1 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=7f9870794f743922aff6caa24e1991d5600b1b8a;p=thirdparty%2Fgit.git test-tool: add helper for name-hash values 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. Since the name-hash values can be stored in the .bitmap files, it is important that these hash functions do not change across Git versions. Add a simple test to t5310-pack-bitmaps.sh to provide some testing of the current values. Due to how these functions are implemented, it would be difficult to change them without disturbing these values. The paths used for this test are carefully selected to demonstrate some of the behavior differences of the two current name hash versions, including which conditions will cause them to collide. 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: distinct hash value: v1 4.1K 5314.3: maximum multiplicity: v1 13 5314.4: distinct hash value: v2 4.2K 5314.5: maximum multiplicity: v2 9 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 19.5K 5314.2: distinct hash value: v1 8.2K 5314.3: maximum multiplicity: v1 279 5314.4: distinct hash value: v2 17.8K 5314.5: maximum multiplicity: v2 44 [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. With the v2 name hash function, the maximum multiplicity lowers to 44, leaving some room for further improvement. In a more extreme example, an internal monorepo had a much worse collision rate: Test this tree -------------------------------------------------- 5314.1: paths at head 227.3K 5314.2: distinct hash value: v1 72.3K 5314.3: maximum multiplicity: v1 14.4K 5314.4: distinct hash value: v2 166.5K 5314.5: maximum multiplicity: v2 138 Here, we can see that the v2 name hash function provides somem improvements, but there are still a number of collisions that could lead to repacking problems at this scale. Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano --- diff --git a/Makefile b/Makefile index 6f5986b66e..65403f6dd0 100644 --- a/Makefile +++ b/Makefile @@ -816,6 +816,7 @@ 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-online-cpus.o TEST_BUILTINS_OBJS += test-pack-mtimes.o TEST_BUILTINS_OBJS += test-parse-options.o diff --git a/t/helper/test-name-hash.c b/t/helper/test-name-hash.c new file mode 100644 index 0000000000..af1d52de10 --- /dev/null +++ b/t/helper/test-name-hash.c @@ -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)) { + printf("%10u ", pack_name_hash(line.buf)); + printf("%10u ", pack_name_hash_v2((unsigned const char *)line.buf)); + printf("%s\n", line.buf); + } + + strbuf_release(&line); + return 0; +} diff --git a/t/helper/test-tool.c b/t/helper/test-tool.c index 1ebb69a5dc..e794058ab6 100644 --- a/t/helper/test-tool.c +++ b/t/helper/test-tool.c @@ -44,6 +44,7 @@ static struct test_cmd cmds[] = { { "match-trees", cmd__match_trees }, { "mergesort", cmd__mergesort }, { "mktemp", cmd__mktemp }, + { "name-hash", cmd__name_hash }, { "online-cpus", cmd__online_cpus }, { "pack-mtimes", cmd__pack_mtimes }, { "parse-options", cmd__parse_options }, diff --git a/t/helper/test-tool.h b/t/helper/test-tool.h index 21802ac27d..26ff30a5a9 100644 --- a/t/helper/test-tool.h +++ b/t/helper/test-tool.h @@ -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); diff --git a/t/perf/p5314-name-hash.sh b/t/perf/p5314-name-hash.sh new file mode 100755 index 0000000000..4ef0ba7711 --- /dev/null +++ b/t/perf/p5314-name-hash.sh @@ -0,0 +1,31 @@ +#!/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 name-hashes +' + +for version in 1 2 +do + test_size "distinct hash value: v$version" ' + awk "{ print \$$version; }" name-hash-count && + wc -l names <<-\EOF && + first + second + third + a/one-long-enough-for-collisions + b/two-long-enough-for-collisions + many/parts/to/this/path/enough/to/collide/in/v2 + enough/parts/to/this/path/enough/to/collide/in/v2 + EOF + + test-tool name-hash out && + + cat >expect <<-\EOF && + 2582249472 1763573760 first + 2289942528 1188134912 second + 2300837888 1130758144 third + 2544516325 3963087891 a/one-long-enough-for-collisions + 2544516325 4013419539 b/two-long-enough-for-collisions + 1420111091 1709547268 many/parts/to/this/path/enough/to/collide/in/v2 + 1420111091 1709547268 enough/parts/to/this/path/enough/to/collide/in/v2 + EOF + + test_cmp expect out +' + test_bitmap_cases () { writeLookupTable=false for i in "$@"