using tree as attribute source is slow, was Re: Help troubleshoot performance regression cloning with depth: git 2.44 vs git 2.42

[Jeff King <peff@peff.net>]

On Tue, Apr 30, 2024 at 10:26:32PM -0700, Dhruva Krishnamurthy wrote:

> Cloning by specifying depth exhibits performance regression in
> pack-objects (~20x). The repository I am cloning is on NFS (mounted
> with NFSv3 & positive lookup cache enabled).
> 
> Ran under 'perf' command to capture profiling information to see if
> something really stands out. There is a significant overhead in calls
> to file open/open64, fstat64 & mmap/munmap in git 2.44 compared to git
> 2.42. Not sure if there is an increase in the number of calls or
> something more is done.
> 
> Could someone please guide me on how to troubleshoot this better?

Oof. I was able to reproduce this regression, and the impact can be
pretty severe. You can reproduce it without NFS and without shallow
clones. Get a bare clone of something big like the kernel:

  git clone --bare https://github.com/torvalds/linux
  cd linux.git

and then do something similar to the server side of a clone:

  time git pack-objects --all --stdout </dev/null >/dev/null

Most of the time will be spent in the "Enumerating objects" phase,
walking the graph (since the repo is fully packed, delta searching and
writing are fairly quick).

With v2.42.0, I get:

  real	1m12.409s
  user	1m13.021s
  sys	0m6.091s

With v2.44.0, I get:

  real	4m12.075s
  user	4m12.763s
  sys	0m5.546s

Bisecting show the culprit is 2386535511 (attr: read attributes from
HEAD when bare repo, 2023-10-13), which is in v2.43.0. Before that, a
bare repository would only look for attributes in the info/attributes
file. But after, we look at the HEAD tree-ish, too. And pack-objects
will check the "delta" attribute for every path of every object we are
packing. And remember that in-tree lookups for foo/bar/baz require
looking not just for .gitattributes, but also foo/.gitattributes,
foo/bar/.gitattributes, and foo/bar/baz/.gitattributes.

In a non-bare repo, this isn't _too_ bad. We'll try an open() for each
path, but the common case is that we don't have such a file, so the
total cost is the one syscall per object. But when we're pulling
attributes from HEAD, each lookup requires walking the whole chain of
trees (so for the final one, tree foo points to tree bar points to tree
baz, where we see there is no .gitattributes entry). And so all of that
extra time goes to reading in trees over and over.

You can repeat the same test with git.git. It gets slower, but it's
barely perceptible. This is because we have a relatively shallow tree,
so most lookups are hitting root-level .gitattributes or maybe one or
two levels deep. The kernel has a much deeper tree, so those lookups are
more expensive (and of course there are a lot more of them).

Getting back to shallow clones and NFS:

  - the effect is more pronounced on NFS because the cost to access
    objects is higher. With git.git I was even able to see some
    slowdown, probably just from the cost/latency of opening up all of
    those objects.

  - the problem doesn't show up if the repo has reachability bitmaps.
    This is because the bitmap result doesn't have the pathnames of each
    object (we do have the "name hash", but it's not enough for us to do
    an attr lookup), and so objects we get from a bitmap do not
    respect the delta attribute at all.

    But when doing a shallow clone, we have to disable bitmaps and do a
    regular traversal. So even if you have bitmaps, you still run into
    the problem.

    The example above should not have bitmaps (we do build them by
    default when repacking bare repos these days, but I don't think
    we'll do so right after cloning). If you have a local repo that
    already has bitmaps, you should be able to see the difference by
    using "git -c pack.usebitmaps=false pack-objects".

    So even if you are a server which generally enables bitmaps, you can
    still get bit by this for shallow clones, but also for other
    non-bitmap invocations, like say "git repack -ad". There I see the
    same 3-minute slowdown in the enumeration phase.

So what to do? It seems like some kind of caching would help here. We're
looking up the same paths over and over, for two reasons:

  1. We'll have many objects with the same paths, one for each time the
     path was modified through history.

  2. Adjacent objects share the higher-level lookups. Both "dir/a" and
     "dir/b" will need to look up "dir/.gitattributes" (and all the way
     up to ".gitattributes").

So even something simple and stupid like this:

diff --git a/attr.c b/attr.c
index 679e42258c..b32af9a78b 100644
--- a/attr.c
+++ b/attr.c
@@ -24,6 +24,7 @@
 #include "thread-utils.h"
 #include "tree-walk.h"
 #include "object-name.h"
+#include "strmap.h"
 
 const char *git_attr_tree;
 
@@ -795,25 +796,31 @@ static struct attr_stack *read_attr_from_blob(struct index_state *istate,
 					      const struct object_id *tree_oid,
 					      const char *path, unsigned flags)
 {
-	struct object_id oid;
-	unsigned long sz;
-	enum object_type type;
+	static struct strmap cache = STRMAP_INIT;
+	void *CACHE_MISSING = (void *)1;
 	void *buf;
-	unsigned short mode;
 
 	if (!tree_oid)
 		return NULL;
 
-	if (get_tree_entry(istate->repo, tree_oid, path, &oid, &mode))
-		return NULL;
-
-	buf = repo_read_object_file(istate->repo, &oid, &type, &sz);
-	if (!buf || type != OBJ_BLOB) {
-		free(buf);
-		return NULL;
+	buf = strmap_get(&cache, path);
+	if (!buf) {
+		struct object_id oid;
+		unsigned short mode;
+		if (!get_tree_entry(istate->repo, tree_oid, path, &oid, &mode)) {
+			unsigned long sz;
+			enum object_type type;
+			buf = repo_read_object_file(istate->repo, &oid, &type, &sz);
+			if (!buf || type != OBJ_BLOB) {
+				free(buf);
+				buf = NULL;
+			}
+		}
+		strmap_put(&cache, path, buf ? buf : CACHE_MISSING);
 	}
 
-	return read_attr_from_buf(buf, path, flags);
+	return buf == CACHE_MISSING ? NULL :
+		read_attr_from_buf(buf, path, flags);
 }
 
 static struct attr_stack *read_attr_from_index(struct index_state *istate,

restores the v2.42 performance. But there are probably better options:

  - this is caching whole .gitattributes buffers. In pack-objects we
    only care about a single bit for try_delta. For linux.git it doesn't
    really matter, as 99% of our entries are just CACHE_MISSING and the
    real value is avoiding the negative lookups. But the same problem
    exists to a lesser degree for "git log -p" in a bare repo. So I
    think it makes sense to try to solve it in the attr layer.

  - the string keys have a lot of duplication. You'll have
    "foo/.gitattributes", "foo/bar/.gitattributes", and so on. A trie
    structure split by path component would let you store each component
    just once. And perhaps have even faster lookups. I think this is
    roughly the same issue faced by the kernel VFS for doing path
    lookups, so something dentry/dcache-like would help.

    I don't know how much it matters in practice, though. The sum of all
    of the paths in HEAD for linux.git is ~3.5MB, which is a rounding
    error on the needs of the rest of the packing process.

  - Something dcache-like could also be pushed down lower, to the
    get_tree_entry() API (imagine it quietly caching uncompressed trees
    behind the scenes and then using them to traverse). That depends on
    high locality of requests, though. I don't know if we have that
    here, because we do our lookups in traversal order. So you'd look at
    entries in HEAD^{tree}, then HEAD~1^{tree}, then HEAD~2^{tree}, and
    so on.

  - Speaking of locality, the attr code tries to make use of request
    locality in its stack (so if we ask for attributes for "foo/bar",
    then "foo/baz", we should be able to keep the parsed data for
    "foo/.gitattributes" available between them). But our pattern here
    violates that. Again, not that big an issue if you don't have that
    many .gitattributes files in the first place, so it might not be
    worth worrying about. But if we did want to rearrange the lookups to
    exploit locality, it might change the overall caching strategy.

  - As noted above, most entries are just CACHE_MISSING. So rather than
    lazily looking up and caching entries, we could just prepopulate the
    cache. And then you know that if an entry isn't present in the
    cache, it does not exist in the tree. The downside is that you pay
    to walk the all of HEAD^{tree}, even if you only have a few lookups
    to do. That's a good tradeoff for pack-objects (which usually ends
    up looking for every path anyway), but not for "git diff" (where you
    only care about a few changed paths.

  - the cache here is static-local in the function. It should probably
    at least be predicated on the tree_oid, and maybe attached to the
    repository object? I think having one per repository at a time would
    be fine (generally the tree_oid is set once per process, so it's not
    like you're switching between multiple options).

I've cc'd John as the author of 2386535511. But really, that was just
enabling by default the attr-tree code added by 47cfc9bd7d (attr: add
flag `--source` to work with tree-ish, 2023-01-14). Although in that
original context (git check-attr) the lack of caching would be much less
important.

-Peff