Re: [PATCH] gc: default aggressive depth to 50

On Thu, Aug 11, 2016 at 12:13:09PM -0400, Jeff King wrote:

quoted

Here are the numbers for linux.git:

   depth |  size |  %    | rev-list |  %     | log -Sfoo |   %
  -------+-------+-------+----------+--------+-----------+-------
    250  | 967MB |  n/a  | 48.159s  |   n/a  | 378.088   |   n/a
    100  | 971MB | +0.4% | 41.471s  | -13.9% | 342.060   |  -9.5%
     50  | 979MB | +1.2% | 37.778s  | -21.6% | 311.040s  | -17.7%
     10  | 1.1GB | +6.6% | 32.518s  | -32.5% | 279.890s  | -25.9%
[...]

You can see that that the CPU savings for regular operations improves as we
decrease the depth. The savings are less for "rev-list" on a smaller repository
than they are for blob-accessing operations, or even rev-list on a larger
repository. This may mean that a larger delta cache would help (though setting
core.deltaBaseCacheLimit by itself doesn't).

The problem with deltaBaseCacheLimit is that it only changes the memory
parameter, but there are a fixed number of slots in the data structure.
Bumping it like this:

diff --git a/sha1_file.c b/sha1_file.c
index 02940f1..ca79703 100644
--- a/sha1_file.c
+++ b/sha1_file.c

@@ -2073,7 +2073,7 @@ 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;

along with the cache size does help (this was discussed a year or two
ago, but nobody ever followed up with numbers or patches).