[PATCH 1/1] upload-pack.c: make output buffer size configurable

[Jacob Vosmaer <jacob@gitlab.com>]

Add a new compile time constant UPLOAD_PACK_BUFFER_SIZE that makes the
size of the static buffer in output_state configurable.

The current size of the buffer is 8192+1. The '+1' is a technical
detail orthogonal to this change. On GitLab.com we use GitLab's
pack-objects cache which does writes of 65515 bytes. Because of the
default 8KB buffer size, propagating these cache writes requires 8
pipe reads and 8 pipe writes from git-upload-pack, and 8 pipe reads
from Gitaly (our Git RPC service). If we increase the size of the
buffer to the maximum Git packet size, we need only 1 pipe read and 1
pipe write in git-upload-pack, and 1 pipe read in Gitaly to transfer
the same amount of data. In benchmarks with a pure fetch and 100%
cache hit rate workload we are seeing CPU utilization reductions of
over 30%.

Signed-off-by: Jacob Vosmaer <jacob@gitlab.com>
---
 upload-pack.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/upload-pack.c b/upload-pack.c
index c78d55bc67..b799fbe628 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -42,6 +42,10 @@
 #define ALL_FLAGS (THEY_HAVE | OUR_REF | WANTED | COMMON_KNOWN | SHALLOW | \
 		NOT_SHALLOW | CLIENT_SHALLOW | HIDDEN_REF)
 
+#ifndef UPLOAD_PACK_BUFFER_SIZE
+#define UPLOAD_PACK_BUFFER_SIZE 8192
+#endif
+
 /* Enum for allowed unadvertised object request (UOR) */
 enum allow_uor {
 	/* Allow specifying sha1 if it is a ref tip. */
@@ -194,7 +198,7 @@ static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
 }
 
 struct output_state {
-	char buffer[8193];
+	char buffer[UPLOAD_PACK_BUFFER_SIZE+1];
 	int used;
 	unsigned packfile_uris_started : 1;
 	unsigned packfile_started : 1;
-- 
2.33.0