Re: [PATCH] fetch: only run 'gc' once when fetching multiple remotes

[Jeff King <peff@peff.net>]

On Wed, Jun 19, 2019 at 04:46:30PM +0700, Nguyễn Thái Ngọc Duy wrote:

> In multiple remotes mode, git-fetch is launched for n-1 remotes and the
> last remote is handled by the current process. Each of these processes
> will in turn run 'gc' at the end.
> 
> This is not really a problem because even if multiple 'gc --auto' is run
> at the same time we still handle it correctly. It does show multiple
> "auto packing in the background" messages though. And we may waste some
> resources when gc actually runs because we still do some stuff before
> checking the lock and moving it to background.
> 
> So let's try to avoid that. We should only need one 'gc' run after all
> objects and references are added anyway. Add a new option --no-auto-gc
> that will be used by those n-1 processes. 'gc --auto' will always run on
> the main fetch process (*).

Yeah, that makes sense.

I was surprised that we needed a new command-line option here, but I
guess the sub-fetch processes really have no idea that they're
subservient to a multi-remote fetch (they do get "--append", but of
course somebody could specify that independently).

Another option would be to just pass "-c gc.auto=0" to the child
processes to inhibit auto-gc. But maybe it makes sense to have a nicer
interface (after all, somebody else could be doing the same "let's do a
bunch of fetches in a row" without using the multi-fetch code).

Though there I kind of wonder if this would apply to other scripted
uses, too. E.g., if I'm doing a bunch of commits, I might want to
inhibit auto-gc and then run it myself at the end. Should we support
"GIT_AUTO_GC=0" in the environment (and a matching "git --no-auto-gc
..." option that could be used here)?

>  Documentation/fetch-options.txt |  4 ++++
>  builtin/fetch.c                 | 17 +++++++++++------
>  t/t5514-fetch-multiple.sh       |  7 +++++--
>  3 files changed, 20 insertions(+), 8 deletions(-)

My musings above aside, the patch looks correct to me.

-Peff