On 19 March 2013 16:01, Johan Tibell <johan.tibell@gmail.com> wrote:
> http://trac.haskell.org/haskell-platform/wiki/Proposals/unordered-containers

The links to the repos are wrong. It should be "tibbe" instead of "tibbel".

Fixed.

Bryan's recent change to change "hashable" to use SipHash is certainly
the right default. There were some complaints about performance for
use cases where security is not an issue. What are the options for
users that wish to use a different hash function? According to the
paper, SipHash is about 2x slower than CityHash.

2x is *a lot*. 2x is about the performance difference between Map and HashMap. Since the raison d'etre for HashMap is that it's faster than Map, if we'd see a 2x slowdown in HashMap there would be little reason to use it.

For example, 'delete' for HashMap ByteString got almost 2x slower with hashable-1.2. Since 'delete' does more than just hashing, that means that SipHash is quite a bit slower than the current (insecure) hash function. Another example: with GHC 7.6.2 HashMap String is almost unusable slow (5x slower than before). This is likely due to a GHC bug, but it's something we need to investigate. At the moment I don't encourage people to upgrade to hashable-1.2.0.5.

The right way to go is probably to make this a user decision. Many applications (e.g. data processing) has no need for the security guarantee so paying for it makes little sense.