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libfetchers/git-utils: Only create pack and mempack backends for the tarball cache

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Now the unnecessary utimensat syscalls from the previous commit
are completely gone:

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ------------------
 33.39    0.646359           3    162898           getdents64
 29.34    0.567866           3    163523     81934 openat
 14.81    0.286739           3     81835       203 newfstatat
 10.98    0.212550           2     81593           close
 10.56    0.204458           2     81544           fstat
  0.15    0.002814           3       870           mmap

The rather crazy amount of getdents64 is still there though.
This commit is contained in:
Sergei Zimmerman 2025-12-02 06:09:03 +03:00
parent 2f6550b7a7
commit 1b2cb1d75c
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2 changed files with 45 additions and 12 deletions
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54
src/libfetchers/git-utils.cc
View file

@ -24,6 +24,7 @@
#include <git2/indexer.h>
#include <git2/object.h>
#include <git2/odb.h>
#include <git2/odb_backend.h>
#include <git2/refs.h>
#include <git2/remote.h>
#include <git2/repository.h>
@ -31,6 +32,7 @@
#include <git2/status.h>
#include <git2/submodule.h>
#include <git2/sys/odb_backend.h>
#include <git2/sys/repository.h>
#include <git2/sys/mempack.h>
#include <git2/tag.h>
#include <git2/tree.h>
@ -245,9 +247,15 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
* In-memory object store for efficient batched writing to packfiles.
* Owned by `repo`.
*/
git_odb_backend * mempack_backend;
git_odb_backend * mempackBackend = nullptr;
GitRepoImpl(std::filesystem::path _path, bool create, bool bare)
/**
* On-disk packfile object store.
* Owned by `repo`.
*/
git_odb_backend * packBackend = nullptr;
GitRepoImpl(std::filesystem::path _path, bool create, bool bare, bool packfilesOnly = false)
: path(std::move(_path))
, bare(bare)
{
@ -258,15 +266,39 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
throw Error("opening Git repository %s: %s", path, git_error_last()->message);
ObjectDb odb;
if (git_repository_odb(Setter(odb), repo.get()))
throw Error("getting Git object database: %s", git_error_last()->message);
if (packfilesOnly) {
/* Create a fresh object database because by default the repo also
loose object backends. We are not using any of those for the
tarball cache, but libgit2 still does a bunch of unnecessary
syscalls that always fail with ENOENT. NOTE: We are only creating
a libgit2 object here and not modifying the repo. Think of this as
enabling the specific backend.
*/
if (git_odb_new(Setter(odb)))
throw Error("creating Git object database: %s", git_error_last()->message);
if (git_odb_backend_pack(&packBackend, (path / "objects").string().c_str()))
throw Error("creating pack backend: %s", git_error_last()->message);
if (git_odb_add_backend(odb.get(), packBackend, 1))
throw Error("adding pack backend to Git object database: %s", git_error_last()->message);
} else {
if (git_repository_odb(Setter(odb), repo.get()))
throw Error("getting Git object database: %s", git_error_last()->message);
}
// mempack_backend will be owned by the repository, so we are not expected to free it ourselves.
if (git_mempack_new(&mempack_backend))
if (git_mempack_new(&mempackBackend))
throw Error("creating mempack backend: %s", git_error_last()->message);
if (git_odb_add_backend(odb.get(), mempack_backend, 999))
if (git_odb_add_backend(odb.get(), mempackBackend, 999))
throw Error("adding mempack backend to Git object database: %s", git_error_last()->message);
if (packfilesOnly) {
if (git_repository_set_odb(repo.get(), odb.get()))
throw Error("setting Git object database: %s", git_error_last()->message);
}
}
operator git_repository *()
@ -287,7 +319,7 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
git_packbuilder_set_threads(packBuilder.get(), 0 /* autodetect */);
packBuilderContext.handleException(
"preparing packfile", git_mempack_write_thin_pack(mempack_backend, packBuilder.get()));
"preparing packfile", git_mempack_write_thin_pack(mempackBackend, packBuilder.get()));
checkInterrupt();
packBuilderContext.handleException("writing packfile", git_packbuilder_write_buf(&buf, packBuilder.get()));
checkInterrupt();
@ -320,7 +352,7 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
if (git_indexer_commit(indexer.get(), &stats))
throw Error("committing git packfile index: %s", git_error_last()->message);
if (git_mempack_reset(mempack_backend))
if (git_mempack_reset(mempackBackend))
throw Error("resetting git mempack backend: %s", git_error_last()->message);
checkInterrupt();
@ -680,9 +712,9 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
}
};
ref<GitRepo> GitRepo::openRepo(const std::filesystem::path & path, bool create, bool bare)
ref<GitRepo> GitRepo::openRepo(const std::filesystem::path & path, bool create, bool bare, bool packfilesOnly)
{
return make_ref<GitRepoImpl>(path, create, bare);
return make_ref<GitRepoImpl>(path, create, bare, packfilesOnly);
}
/**
@ -1361,7 +1393,7 @@ namespace fetchers {
ref<GitRepo> Settings::getTarballCache() const
{
static auto repoDir = std::filesystem::path(getCacheDir()) / "tarball-cache";
return GitRepo::openRepo(repoDir, true, true);
return GitRepo::openRepo(repoDir, /*create=*/true, /*bare=*/true, /*packfilesOnly=*/true);
}
} // namespace fetchers

3
src/libfetchers/include/nix/fetchers/git-utils.hh
View file

@ -32,7 +32,8 @@ struct GitRepo
{
virtual ~GitRepo() {}
static ref<GitRepo> openRepo(const std::filesystem::path & path, bool create = false, bool bare = false);
static ref<GitRepo>
openRepo(const std::filesystem::path & path, bool create = false, bool bare = false, bool packfilesOnly = false);
virtual uint64_t getRevCount(const Hash & rev) = 0;