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From: Jeff King <peff@peff.net>
To: John Cai <johncai86@gmail.com>
Cc: git <git@vger.kernel.org>, Christian Couder <christian.couder@gmail.com>
Subject: [PATCH 2/3] fsck: turn off save_commit_buffer
Date: Thu, 22 Sep 2022 06:13:36 -0400	[thread overview]
Message-ID: <Yyw1UIhhj5zVtl1T@coredump.intra.peff.net> (raw)
In-Reply-To: <Yyw0PSVe3YTQGgRS@coredump.intra.peff.net>

When parsing a commit, the default behavior is to stuff the original
buffer into a commit_slab (which takes ownership of it). But for a tool
like fsck, this isn't useful. While we may look at the buffer further as
part of fsck_commit(), we'll always do so through a separate pointer;
attaching the buffer to the slab doesn't help.

Worse, it means we have to remember to free the commit buffer in all
call paths. We do so in fsck_obj(), which covers a regular "git fsck".
But with "--connectivity-only", we forget to do so in both
traverse_one_object(), which covers reachable objects, and
mark_unreachable_referents(), which covers unreachable ones. As a
result, that mode ends up storing an uncompressed copy of every commit
on the heap at once.

We could teach the code paths for --connectivity-only to also free
commit buffers. But there's an even easier fix: we can just turn off the
save_commit_buffer flag, and then we won't attach them to the commits in
the first place.

This reduces the peak heap of running "git fsck --connectivity-only" in
a clone of linux.git from ~2GB to ~1GB. According to massif, the
remaining memory goes where you'd expect: the object structs themselves,
the obj_hash containing them, and the delta base cache.

Note that we'll leave the call to free commit buffers in fsck_obj() for
now; it's not quite redundant because of a related bug that we'll fix in
a subsequent commit.

Signed-off-by: Jeff King <peff@peff.net>
---
 builtin/fsck.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/builtin/fsck.c b/builtin/fsck.c
index 34e575a170..b45de003d4 100644
--- a/builtin/fsck.c
+++ b/builtin/fsck.c
@@ -855,6 +855,7 @@ int cmd_fsck(int argc, const char **argv, const char *prefix)
 
 	errors_found = 0;
 	read_replace_refs = 0;
+	save_commit_buffer = 0;
 
 	argc = parse_options(argc, argv, prefix, fsck_opts, fsck_usage, 0);
 
-- 
2.38.0.rc1.583.ga560cd8328


  parent reply	other threads:[~2022-09-22 10:14 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-20 19:27 [INVESTIGATION] why is fsck --connectivity-only so much more expensive than rev-list --objects --all? John Cai
2022-09-20 20:41 ` Jeff King
2022-09-22 10:09   ` [PATCH 0/3] reducing fsck memory usage Jeff King
2022-09-22 10:11     ` [PATCH 1/3] fsck: free tree buffers after walking unreachable objects Jeff King
2022-09-22 18:40       ` Junio C Hamano
2022-09-22 18:58         ` Jeff King
2022-09-22 19:27           ` Junio C Hamano
2022-09-22 22:16             ` Jeff King
2022-09-22 10:13     ` Jeff King [this message]
2022-09-22 10:15     ` [PATCH 3/3] parse_object_buffer(): respect save_commit_buffer Jeff King

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