Hi Thomas, On Tue, Mar 19, 2013 at 8:41 AM, Thomas Schilling <nomin...@googlemail.com>wrote:
> On 19 March 2013 16:01, Johan Tibell <johan.tib...@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. Cheers, Johan
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