Add cache preload facility

This can do the lstat() storm in parallel, giving potentially much
improved performance for cold-cache cases or things like NFS that have
weak metadata caching.

Just use "read_cache_preload()" instead of "read_cache()" to force an
optimistic preload of the index stat data.  The function takes a
pathspec as its argument, allowing us to preload only the relevant
portion of the index.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 32dcd643d2..0d96e763ac 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -413,6 +413,15 @@ data writes properly, but can be useful for filesystems that do not use
 journalling (traditional UNIX filesystems) or that only journal metadata
 and not file contents (OS X's HFS+, or Linux ext3 with "data=writeback").
 
+core.preloadindex::
+	Enable parallel index preload for operations like 'git diff'
++
+This can speed up operations like 'git diff' and 'git status' especially
+on filesystems like NFS that have weak caching semantics and thus
+relatively high IO latencies.  With this set to 'true', git will do the
+index comparison to the filesystem data in parallel, allowing
+overlapping IO's.
+
 alias.*::
 	Command aliases for the linkgit:git[1] command wrapper - e.g.
 	after defining "alias.last = cat-file commit HEAD", the invocation