[PATCH] Teach 'git checkout' to preload the index contents

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: May 9, 2009, 22:41 UTC
Message-ID: <alpine.LFD.2.01.0905091517000.3586@localhost.localdomain>
In-Reply-To: <alpine.LFD.2.01.0905091501460.3586@localhost.localdomain>

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sat, 9 May 2009 15:11:17 -0700

This makes git checkout know to use the threaded index preloading if it is enabled in the config file. You need to have

	[core]
		preloadindex = true

in your config file to see it, and for that feature to make sense your filesystem needs to be able to do concurrent 'lstat()' lookups, but when that is the case (especially NFS over a high-latency network), this can be a noticeable performance win.

But with a low-latency network and at least older Linux NFS clients, this will clearly potentially cause a lot of lock contention. It may still speed up the uncached case, but the threading and locking overhead will result in the cached case likely slowing down.

That was almost certainly fixed by Linux commit fc0f684c2 ("NFS: Remove BKL from NFS lookup code"), but that one got merged into 2.6.27-rc1, so older kernel versions than 2.6.27 will not scale very well.

But regardless, it's the right thing to do. If your filesystem doesn't scale, don't enable index preloading.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---

On local filesystems, I've actually seen signs of tiny speedups (iow, 0.15s -> 0.14s in the cached case on a much-too-fast-for-this-to-matter Nehalem machine) due to SMP scaling.

But it's probably more likely to slow things down, unless you have a disk that does TCQ and really gets improved by having multiple outstanding requests.

Again, this is not so much a "git checkout" issue, as a generic "core.preloadindex" issue.

 builtin-checkout.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/builtin-checkout.c b/builtin-checkout.c
index 15f0c32..3100ccd 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -216,7 +216,7 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
 
 	newfd = hold_locked_index(lock_file, 1);
-	if (read_cache() < 0)
+	if (read_cache_preload(pathspec) < 0)
 		return error("corrupt index file");
 
 	if (source_tree)
@@ -367,7 +367,7 @@ static int merge_working_tree(struct checkout_opts *opts,
 	int newfd = hold_locked_index(lock_file, 1);
 	int reprime_cache_tree = 0;
 
-	if (read_cache() < 0)
+	if (read_cache_preload(NULL) < 0)
 		return error("corrupt index file");
 
 	cache_tree_free(&active_cache_tree);

-- 
1.6.3.1.g3f31