Re: [PATCH 4/4] gc --aggressive: three phase repacking

[Jeff King <peff@peff.net>]

On Tue, Mar 18, 2014 at 12:50:50AM -0400, Jeff King wrote:

> On Sun, Mar 16, 2014 at 08:35:04PM +0700, Nguyễn Thái Ngọc Duy wrote:
> 
> > As explained in the previous commit, current aggressive settings
> > --depth=250 --window=250 could slow down repository access
> > significantly. Notice that people usually work on recent history only,
> > we could keep recent history more loosely packed, so that repo access
> > is fast most of the time while the pack file remains small.
> 
> One thing I have not seen is real-world timings showing the slowdown
> based on --depth. Did I miss them, or are we just making assumptions
> based on one old case from 2009 (that, AFAIK does not have real numbers,
> just speculation)? Has anyone measured the effect of bumping the delta
> cache size (and its hash implementation)?

Just as a very quick, rough data point, here are before-and-after
timings for the patch below doing "git rev-list --objects --all" on my
linux.git, which is a mix of "--aggressive" and normal packing (I didn't
do a "repack -f", but it's partially what I've downloaded from k.org and
what I've repacked in various experiments over the past few months).

  [before]
  real    0m28.824s
  user    0m28.620s
  sys     0m0.232s

  [after]
  real    0m21.694s
  user    0m21.544s
  sys     0m0.172s

The numbers below are completely pulled out of a hat, so we can perhaps
do even better. But I think it shows that there is room for improvement
in the delta base cache.

---
diff --git a/environment.c b/environment.c
index c3c8606..73ed670 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 = 128 * 1024 * 1024;
 unsigned long big_file_threshold = 512 * 1024 * 1024;
 const char *pager_program;
 int pager_use_color = 1;
diff --git a/sha1_file.c b/sha1_file.c
index b37c6f6..a9ab8e3 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -1944,7 +1944,7 @@ static void *unpack_compressed_entry(struct packed_git *p,
 	return buffer;
 }
 
-#define MAX_DELTA_CACHE (256)
+#define MAX_DELTA_CACHE (1024)
 
 static size_t delta_base_cached;