Re: hashmap vs khash? Re: [PATCH] packfile.c: speed up loading lots of packfiles.

From: Jeff King <peff@peff.net>
Date: Dec 2, 2019, 14:39 UTC
Message-ID: <20191202143944.GA18276@sigill.intra.peff.net>
In-Reply-To: <20191128004202.GA25910@dcvr>

On Thu, Nov 28, 2019 at 12:42:02AM +0000, Eric Wong wrote:

Show 9 quoted lines

> > Add a hashmap containing the packfile names as we load them so that
> > the average runtime cost of checking for already-loaded packs becomes
> > constant.
> 
> Btw, would you have time to do a comparison against khash?
> 
> AFAIK hashmap predates khash in git; and hashmap was optimized
> for removal.   Removals don't seem to be a problem for pack
> loading.

Actually, they came around simultaneously. I think hashmap.[ch] was mostly a response to our open-coded hashes, like the one in object.c (which still uses neither of the reusable forms!). Those didn't handle removal at all. khash does handle removal, though you pay a price in tombstone entries until the next resize operation.

> I'm interested in exploring the removing of hashmap entirely in
> favor of khash to keep our codebase smaller and easier-to-learn.
> khash shows up more in other projects, and ought to have better
> cache-locality.

I have been tempted to push for that, too. Every timing I have ever done shows khash as faster (though for a trivial use like this one, I would be quite surprised if it mattered either way).

My hesitation is that khash can be harder to debug because of the macro implementation. But I have rarely needed to look beneath its API.

-Peff