Re: [PATCH] parse_object: try internal cache before reading object db

From: Junio C Hamano <gitster@pobox.com>
Date: Jan 5, 2012, 21:35 UTC
Message-ID: <7vipkpn87d.fsf@alter.siamese.dyndns.org>
In-Reply-To: <20120105210001.GA30549@sigill.intra.peff.net>

Jeff King <peff@peff.net> writes:

Show 7 quoted lines

> This might seem circular, since step 2a uses the type
> information determined in step 1 to call the appropriate
> lookup function. However, we can notice that all of the
> lookup_* functions are backed by lookup_object. In other
> words, all of the objects are kept in a master hash table,
> and we don't actually need the type to do the "do we have
> it" part of the lookup,...

The only case that might matter is where you read one object, you have written another object of a different type but that happens to hash to the same SHA-1 value. The other existing optimizations do not take that into account, so I do not think there is any new issue here.

> For example, GitHub's alternates repository for git.git has
> ~120,000 refs, of which only ~3200 are unique. The time for
> upload-pack to print its list of advertised refs dropped
> from 3.4s to 0.76s.

Nice. I am more impressed by 120k/3.4 than 3.2k/0.76, though ;-)

Thanks.