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upload-pack.c: increase output buffer size 

When serving a fetch, git upload-pack copies data from a git
pack-objects stdout pipe to its stdout. This commit increases the size
of the buffer used for that copying from 8192 to 65515, the maximum
sideband-64k packet size.

Previously, this buffer was allocated on the stack. Because the new
buffer size is nearly 64KB, we switch this to a heap allocation.

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Jacob Vosmaer 2021-12-14 20:46:26 +01:00 committed by Junio C Hamano
parent 69a9c10c95
commit 55a9651d26
1 changed files with 12 additions and 5 deletions
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17
upload-pack.c
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@ -194,7 +194,13 @@ static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
}
struct output_state {
char buffer[8193];
/*
* We do writes no bigger than LARGE_PACKET_DATA_MAX - 1, because with
* sideband-64k the band designator takes up 1 byte of space. Because
* relay_pack_data keeps the last byte to itself, we make the buffer 1
* byte bigger than the intended maximum write size.
*/
char buffer[(LARGE_PACKET_DATA_MAX - 1) + 1];
int used;
unsigned packfile_uris_started : 1;
unsigned packfile_started : 1;
@ -269,7 +275,7 @@ static void create_pack_file(struct upload_pack_data *pack_data,
const struct string_list *uri_protocols)
{
struct child_process pack_objects = CHILD_PROCESS_INIT;
struct output_state output_state = { { 0 } };
struct output_state *output_state = xcalloc(1, sizeof(struct output_state));
char progress[128];
char abort_msg[] = "aborting due to possible repository "
"corruption on the remote side.";
@ -404,7 +410,7 @@ static void create_pack_file(struct upload_pack_data *pack_data,
}
if (0 <= pu && (pfd[pu].revents & (POLLIN|POLLHUP))) {
int result = relay_pack_data(pack_objects.out,
&output_state,
output_state,
pack_data->use_sideband,
!!uri_protocols);
@ -438,11 +444,12 @@ static void create_pack_file(struct upload_pack_data *pack_data,
}
/* flush the data */
if (output_state.used > 0) {
send_client_data(1, output_state.buffer, output_state.used,
if (output_state->used > 0) {
send_client_data(1, output_state->buffer, output_state->used,
pack_data->use_sideband);
fprintf(stderr, "flushed.\n");
}
free(output_state);
if (pack_data->use_sideband)
packet_flush(1);
return;