Re: Warnings in gc.log can prevent gc --auto from running

From: Jeff King <peff@peff.net>
Date: Jul 29, 2019, 10:07 UTC
Message-ID: <20190729100745.GA2755@sigill.intra.peff.net>
In-Reply-To: <qhdnuh$5m5r$1@blaine.gmane.org>

On Thu, Jul 25, 2019 at 07:18:57PM -0700, Gregory Szorc wrote:

Show 7 quoted lines

> I think I've found some undesirable behavior with regards to the
> behavior of `git gc --auto`. The tl;dr is that a warning message written
> to gc.log can result in `git gc --auto` effectively disabling itself for
> gc.logExpiry. The problem is easier to trigger in 2.22 as a result of
> enabling bitmap indices for bare repositories by default and the
> behavior can easily result in performance degradation, especially on
> servers.

Yuck, thanks for reporting this.

As you note, this is a special case of a much larger problem. The other common case is the "oops, you still have a lot of loose objects after repacking" warning. There's more discussion and some patches here:

  https://public-inbox.org/git/20180716172717.237373-1-jonathantanmy@google.com/

though I don't think any of the work that came out of that fundamentally solves the issue.

Show 5 quoted lines

> I don't prescribe to know the best way to solve this problem. I just
> know it is a footgun sitting in the default Git configuration. And the
> footgun became a lot easier to fire with the introduction of warning
> messages related to bitmap indices and again when bitmap indices were
> enabled by default for bare repositories in Git 2.22.

IMHO one way to mitigate this is to simply warn less. In particular, if we are auto-enabling bitmaps, then it doesn't necessarily make sense for us to warn about them being disabled.

In the case of .keep files, we've already got 7328482253 (repack: disable bitmaps-by-default if .keep files exist, 2019-06-29), which should be in the next released version of Git. But I suspect that's racy with respect to somebody creating .keep files, and as you note there are other config options that might prevent us from generating bitmaps.

Instead, it may make sense to turn the --write-bitmap-index option of pack-objects into a tri-state: true/false/auto. Then pack-objects would know that we are in best-effort mode, and would avoid warning in that case. That would also let git-repack express its intentions better to git-pack-objects, so we could replace 7328482253, and keep more of the logic in pack-objects, which is ultimately what has to make the decision about whether it can generate bitmaps.

-Peff