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From: Jeff King <>
To: Eric Wong <>
Cc: Colin Stolley <>,
Subject: Re: hashmap vs khash? Re: [PATCH] packfile.c: speed up loading lots of packfiles.
Date: Mon, 2 Dec 2019 09:39:44 -0500
Message-ID: <> (raw)
In-Reply-To: <20191128004202.GA25910@dcvr>

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

> > 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.


  parent reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-27 22:24 Colin Stolley
2019-11-28  0:42 ` hashmap vs khash? " Eric Wong
2019-11-30 17:36   ` Junio C Hamano
2019-12-02 14:39   ` Jeff King [this message]
2019-12-02 17:40 ` SZEDER Gábor
2019-12-02 19:42   ` Jeff King
2019-12-03  6:17     ` Taylor Blau
2019-12-03 15:34       ` Jeff King
2019-12-03 16:04     ` Junio C Hamano
2019-12-03 17:33       ` Colin Stolley
2019-12-03 22:18         ` Jeff King
2019-12-04 18:15           ` Junio C Hamano
2019-12-03 22:17       ` Jeff King
2019-12-04  4:23         ` Jonathan Nieder
2019-12-03  6:19 ` Taylor Blau

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