From: Jeff King <peff@peff.net>
Date: Fri, 01 Nov 2024 17:45:47 GMT
Subject: Re: [PATCH v4 8/9] config: make `packed_git_(limit|window_size)` non-global variables
Message-ID: <20241101174547.GB2337295@coredump.intra.peff.net>
In-Reply-To: <5bbdc7124d58526a7a2d7b3bdc807ddd204a6df1.1730366765.git.karthik.188@gmail.com>

On Thu, Oct 31, 2024 at 10:39:51AM +0100, Karthik Nayak wrote:

> @@ -652,20 +688,25 @@ unsigned char *use_pack(struct packed_git *p,
>  				break;
>  		}
>  		if (!win) {
> -			size_t window_align = packed_git_window_size / 2;
> +			struct packfile_config config = PACKFILE_CONFIG_INIT;
> +			size_t window_align;
>  			off_t len;
>  
> +			repo_config(p->repo, packfile_config, &config);
> +			window_align = config.packed_git_window_size / 2;
> +

Parsing config like this is somewhat expensive (remember we're going to
hit your callback for every single config key in the system, user, and
repo-level config files).

And use_pack() is a relatively hot code path, as we call it any time we
need to access bytes from a mapped pack! This "!win" conditional isn't
quite as hot, as it only triggers when we establish a new window for a
pack. But that still happens at least once per pack, more if we need to
move the window around in a big pack, and lots more if we are under
memory pressure and need to open/close windows a lot.

I think we need to parse these values once and then store them somewhere
with cheaper access. Can we grab them in prepare_repo_settings(), for
example, which would cache them? We need a repo struct, but we have one
(the same packed_git->repo you are using to call repo_config()).

-Peff