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

Jeff King [off-list ref] 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/