From: Taylor Blau <me@ttaylorr.com>
Date: Mon, 19 Oct 2020 17:37:24 GMT
Subject: Re: [PATCH 1/4] fsmonitor: use fsmonitor data in `git diff`
Message-ID: <20201019173724.GA42706@nand.local>
In-Reply-To: <xmqqr1puuo35.fsf@gitster.c.googlers.com>

On Mon, Oct 19, 2020 at 10:23:26AM -0700, Junio C Hamano wrote:
> Taylor Blau <me@ttaylorr.com> writes:
>
> >> > There is some overhead to invoke the hook and talk to watchman, but
> >> > I'd expect that to be dwarfed by not having to issue O(# files)
> >> > syscalls.
> >>
> >> "invoke the hook"---is that a pipe+fork+exec, or something else that
> >> is far lighter-weight?
> >
> > The former; see 'fsmonitor.c:query_fsmonitor()'.
>
> It brings us back to the "overhead of how many lstat(2) takes us
> closer to the overhead of a single pipe+fork+exec plus reading from
> the pipe", doesn't it?

Somewhat unfortunately, yes. Hopefully any user that cares to use
fsmonitor has enough files in their repository that a pipe+fork+exec is
still faster than however many lstats they would have needed otherwise.

Of course, finding out what that number is is still interesting...

Thanks,
Taylor