Subject: [PATCH] Bump core.deltaBaseCacheLimit to 96m

From: David Kastrup <[email-masked]>
Date: Sun, 4 May 2014 19:13:57 +0200

The default of 16m causes serious thrashing for large delta chains
combined with large files.

Here are some benchmarks (pu variant of git blame):

time git blame -C src/xdisp.c >/dev/null

for a repository of Emacs repacked with git gc --aggressive (v1.9,
resulting in a window size of 250) located on an SSD drive.  The file in
question has about 30000 lines, 1Mb of size, and a history with about
2500 commits.

16m (previous default):
real	3m33.936s
user	2m15.396s
sys	1m17.352s

32m:
real	3m1.319s
user	2m8.660s
sys	0m51.904s

64m:
real	2m20.636s
user	1m55.780s
sys	0m23.964s

96m:
real	2m5.668s
user	1m50.784s
sys	0m14.288s

128m:
real	2m4.337s
user	1m50.764s
sys	0m12.832s

192m:
real	2m3.567s
user	1m49.508s
sys	0m13.312s

Signed-off-by: David Kastrup <[email-masked]>
---
 Documentation/config.txt | 2 +-
 environment.c            | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 1932e9b..21a3c86 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -489,7 +489,7 @@ core.deltaBaseCacheLimit::
 	to avoid unpacking and decompressing frequently used base
 	objects multiple times.
 +
-Default is 16 MiB on all platforms.  This should be reasonable
+Default is 96 MiB on all platforms.  This should be reasonable
 for all users/operating systems, except on the largest projects.
 You probably do not need to adjust this value.
 +
diff --git a/environment.c b/environment.c
index 5c4815d..37354c8 100644
--- a/environment.c
+++ b/environment.c
@@ -37,7 +37,7 @@ int core_compression_seen;
 int fsync_object_files;
 size_t packed_git_window_size = DEFAULT_PACKED_GIT_WINDOW_SIZE;
 size_t packed_git_limit = DEFAULT_PACKED_GIT_LIMIT;
-size_t delta_base_cache_limit = 16 * 1024 * 1024;
+size_t delta_base_cache_limit = 96 * 1024 * 1024;
 unsigned long big_file_threshold = 512 * 1024 * 1024;
 const char *pager_program;
 int pager_use_color = 1;
-- 
1.9.1