Re: [PATCH] xdiff: Do not enable XDL_FAST_HASH by default

[Junio C Hamano <gitster@pobox.com>]

Jeff King <peff@peff.net> writes:

> This is a nice incremental step in the sense that people can still
> enable it if they want to in order to time it, play with it, etc. But
> given what we know, I wonder if the help text here should warn people.
>
> Or I guess we could move straight to dropping it entirely.
>
> Here's what that patch might look like (I retimed it just be sure, and
> was sad to see that it really _is_ making some cases faster. But I still
> think slower-but-predictable is a better default).

I like this version that drops quite a lot of code ;-)

> Subject: [PATCH] xdiff: drop XDL_FAST_HASH
> ...
> The idea of XDL_FAST_HASH is to speed up the hash
> computation. But the generated hashes have worse collision
> behavior. This means that in some cases it speeds diffs up
> (running "git log -p" on git.git improves by ~8% with it),
> but in others it can slow things down. One pathological case
> saw over a 100x slowdown[1].
>
> There may be a better hash function that covers both
> properties, but in the meantime we are better off with the
> original hash. It's slightly slower in the common case, but
> it has fewer surprising pathological cases.
>
> [1] http://public-inbox.org/git/20141222041944.GA441@peff.net/