Subject: Git fetch slow on local repository with 600k refs

From: 程洋 <[email-masked]>
Date: Mon, 13 Mar 2023 11:54:23 +0000

We're holding a Gerrit server cluster. And uses pull-replication plugin to sync changes between master and slave.
When a change is pushed to master, it notify the slave, and slave fetch it from master.

But we found in a big repository with 600k refs. Fetch takes 5-10 seconds even if fetching a 1 byte change. Here is the GIT_TRACE2_PERF
I did an experiment to fetch a ref that my slave already have. And we can find git rev-list takes 2 seconds to perform. (I guess it try to find remote object from reachable objects of local refs one by one)
Is there anyway to optimize such situation?

19:12:55.931180 common-main.c:48             | d0 | main                     | version      |     |           |           |              | 2.33.1.558.g2bd2f258f4.dirty
19:12:55.931215 common-main.c:49             | d0 | main                     | start        |     |  0.000335 |           |              | git fetch --no-tags git://10.13.8.10/miui/gerrit/base-test.git refs/changes/27/2741927/1:refs/changes/27/2741927/1
19:12:55.931302 compat/linux/procinfo.c:170  | d0 | main                     | cmd_ancestry |     |           |           |              | ancestry:[bash sudo bash miauthd miauthd systemd]
19:12:55.931381 git.c:456                    | d0 | main                     | cmd_name     |     |           |           |              | fetch (fetch)
19:12:55.931566 builtin/fetch.c:1579         | d0 | main                     | region_enter | r0  |  0.000692 |           | fetch        | label:remote_refs
19:12:55.936781 connect.c:167                | d0 | main                     | data         |     |  0.005907 |  0.005215 | transfer     | ..negotiated-version:2
19:12:55.940447 builtin/fetch.c:1582         | d0 | main                     | region_leave | r0  |  0.009573 |  0.008881 | fetch        | label:remote_refs
19:12:56.221133 run-command.c:739            | d0 | main                     | child_start  |     |  0.290252 |           |              | [ch0] class:? argv:[git rev-list --objects --stdin --not --all --quiet --alternate-refs --unsorted-input]
19:12:58.014792 run-command.c:995            | d0 | main                     | child_exit   |     |  2.083899 |  1.793647 |              | [ch0] pid:81860 code:0
19:12:58.014855 builtin/fetch.c:1321         | d0 | main                     | region_enter | r0  |  2.083980 |           | fetch        | label:consume_refs
19:12:58.015412 builtin/fetch.c:1326         | d0 | main                     | region_leave | r0  |  2.084538 |  0.000558 | fetch        | label:consume_refs
19:12:58.015466 run-command.c:739            | d0 | main                     | child_start  |     |  2.084590 |           |              | [ch1] class:? argv:[git maintenance run --auto --no-quiet]
19:12:58.018879 common-main.c:48             | d1 | main                     | version      |     |           |           |              | 2.33.1.558.g2bd2f258f4.dirty
19:12:58.018911 common-main.c:49             | d1 | main                     | start        |     |  0.000324 |           |              | /usr/libexec/git-core/git maintenance run --auto --no-quiet
19:12:58.019011 compat/linux/procinfo.c:170  | d1 | main                     | cmd_ancestry |     |           |           |              | ancestry:[git bash sudo bash miauthd miauthd systemd]
19:12:58.019087 git.c:456                    | d1 | main                     | cmd_name     |     |           |           |              | maintenance (fetch/maintenance)
19:12:58.019276 git.c:714                    | d1 | main                     | exit         |     |  0.000690 |           |              | code:0
19:12:58.019284 trace2/tr2_tgt_perf.c:213    | d1 | main                     | atexit       |     |  0.000698 |           |              | code:0
19:12:58.019386 run-command.c:995            | d0 | main                     | child_exit   |     |  2.088507 |  0.003917 |              | [ch1] pid:81878 code:0
19:12:58.019411 git.c:714                    | d0 | main                     | exit         |     |  2.088538 |           |              | code:0
19:12:58.019419 trace2/tr2_tgt_perf.c:213    | d0 | main                     | atexit       |     |  2.088545 |           |              | code:0
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