Re: [PATCH 5/9] handle alternates paths the same as the main object dir

[Jeff King <peff@peff.net>]

On Mon, Nov 12, 2018 at 10:38:28AM -0500, Derrick Stolee wrote:

> > We could go either direction here, but this patch moves the alternates
> > struct over to the main directory style (rather than vice-versa).
> > Technically the alternates style is more efficient, as it avoids
> > rewriting the object directory name on each call. But this is unlikely
> > to matter in practice, as we avoid reallocations either way (and nobody
> > has ever noticed or complained that the main object directory is copying
> > a few extra bytes before making a much more expensive system call).
> 
> Hm. I've complained in the past [1] about a simple method like strbuf_addf()
> over loose objects, but that was during abbreviation checks so we were
> adding the string for every loose object but not actually reading the
> objects.
> 
> [1]
> https://public-inbox.org/git/20171201174956.143245-1-dstolee@microsoft.com/

I suspect that had more to do with the cost of snprintf() than the extra
bytes being copied. And here we'd still be using addstr and addch
exclusively. I'm open to numeric arguments to the contrary, though. :)

There's actually a lot of low-hanging fruit there for pre-sizing, too.
E.g., fill_sha1_path() calls strbuf_addch() in a loop, but it could
quite easily grow the 41 bytes it needs ahead of time. I wouldn't want
to change that without finding a measurable improvement, though. It
might not be a big deal due to fec501dae8 (strbuf_addch: avoid calling
strbuf_grow, 2015-04-16).

-Peff