Avoid reopening packfile on every object access #852
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I tested this against Here is the 
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jtibshirani
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| // It copies the relevant logic from git.PlainOpen, and enables the filesystem KeepDescriptors option. This | ||
| // caches the packfile handles, preventing the packfile from being opened then closed on every object access. | ||
| func openRepo(repoDir string) (*git.Repository, io.Closer, error) { | ||
| fs := osfs.New(repoDir) |
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I included all the parts from git.PlainOpen that seem relevant. I also checked the go-git examples for KeepDescriptors, and this is almost identical: https://github.com/go-git/go-git/blob/3a210b5157e7d2a57eeac29e492efa510d616a24/_examples/ls/main.go#L32-L42
However git.PlainOpen is quite complex and I could be missing something. Here's what I purposefully omitted:
- Special handling for user home shortcuts ... I tested and this was not necessary 🤔
- Detecting when
.gitis a file that points to another git directory (like if you're trying to directly index submodule directory). I don't think we do this in indexing, or ever would.
I've tested zoekt-git-index with all the following:
- Repos with and without
.gitsubdirectories - Relative paths like
../megarepo - Shortcut for user home like
~/code/megarepo - Updating Sourcegraph to use this version and reindexing a bunch of repos locally
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| s := filesystem.NewStorageWithOptions(fs, cache.NewObjectLRUDefault(), filesystem.Options{ | ||
| KeepDescriptors: true, |
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Here's the point of the whole change :) This also sets me up to set LargeObjectThreshold in a follow-up PR, which could prove even more important.
This command is great, thanks for the pointer. My interpretation of the results below:
Before After |
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Latest changes:
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| executeCommand(t, repoDir, exec.Command("git", "config", "user.name", "Thomas")) | ||
| executeCommand(t, repoDir, exec.Command("git", "config", "user.email", "thomas@google.com")) |
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This is a nice way to do it in a repo. FYI in the sourcegraph repo what we often do is set these envvars, which also allow you to get deterministic commits since it controls the timestamps:
c.Env = []string{
"GIT_CONFIG=" + path.Join(dir, ".git", "config"),
"GIT_COMMITTER_NAME=a",
"GIT_COMMITTER_EMAIL=a@a.com",
"GIT_COMMITTER_DATE=2006-01-02T15:04:05Z",
"GIT_AUTHOR_NAME=a",
"GIT_AUTHOR_EMAIL=a@a.com",
"GIT_AUTHOR_DATE=2006-01-02T15:04:05Z",
}
The setting of GIT_CONFIG is useful for the init command, since then it also avoids reading the users git config which can affect things.
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Thanks for the pointer, this is nicer.
By default go-git maintains an LRU cache of git objects of size 96MB. When an object's contents are loaded, it's stored as a MemoryObject in this cache. This cache is not super useful in the indexing access pattern, which accesses each file only once. And in many profiles, we see a substantial number of allocations from these memory objects. This PR disables caching for most git objects by setting LargeObjectThreshold: 1. go-git still proactively caches packfile objects under 16KB (see smallObjectThreshold here). Follow up to #852. This change is also gated by the ZOEKT_ENABLE_GOGIT_OPTIMIZATION feature flag.
By default, the
go-gitlibrary will open the packfile on every call toRepository.BlobObject, then close it. During indexing, we collect the list of files to index, then iterate through each one callingRepository.BlobObject. So on every object access the packfile reopened, andgo-gitreallocates some in-memory buffers.This PR bypasses
git.PlainOpento allow us to enable theKeepDescriptorsoption. This option keeps packfile files open, and caches wrappers for them. The files then need to be explicitly closed when done with the repo.Benefits:
Relates to SPLF-636