[infinispan-dev] Consistent hashes and Hot Rod

[Dan Berindei]

We will still have a division in the consistent hash, because the "number
of segments" in some ways replaces . We just didn't add it to the HotRod
protocol yet...

I have thought about replacing division, and I was considering 2 options:
1. Hide the power of 2 restriction in the consistent hash implementation -
allow the user to specify any number of segments, but round it up
internally.
2. Compute the multiplicative inverse of the number of segments and use
multiplication instead - multiplication should be just as fast as addition
on modern machines.

However, the division didn't particularly stick out as a hotspot back when
I implemented DefaultConsistentHash, so I didn't pursue it further.

Cheers
Dan


On Thu, Aug 29, 2013 at 5:29 PM, Manik Surtani <msurtani at redhat.com> wrote:

> Pls ignore.  I've just been informed that hash space is to be dropped from
> the protocol anyway.
>
> On 29 Aug 2013, at 13:35, Manik Surtani <msurtani at redhat.com> wrote:
>
> > The Hot Rod protocol current passes the size of the hash space used to
> clients, so that clients are able to create a consistent hash and perform
> smart routing.
> >
> >
> https://docs.jboss.org/author/display/ISPN/Hot+Rod+Protocol+-+Version+1.0#HotRodProtocol-Version1.0-HashDistributionAwareClientTopologyChangeHeader
> >
> https://docs.jboss.org/author/display/ISPN/Hot+Rod+Protocol+-+Version+1.2#HotRodProtocol-Version1.2-HashDistributionAwareClientTopologyChangeHeader
> >
> > However there are no rules as to what this hash space is, and this
> forces clients to use the remainder operation when performing modular
> arithmetic.  This has been proven to be inefficient [1] [2] [3], and can be
> greatly improved if we made a simple assumption that the hash space will
> always be a power of two.
> >
> > Perhaps something we can add in v1.3 of the protocol, so that clients
> talking to a server using Hot Rod 1.3 can make the assumption that the hash
> space is a power of 2?
> >
> > WDYT?
> >
> > - Manik
> >
> >
> > [1]
> http://dhruba.name/2011/07/12/performance-pattern-modulo-and-powers-of-two/
> > [2]
> http://scicomp.stackexchange.com/questions/187/why-is-division-so-much-more-complex-than-other-arithmetic-operations
> > [3] http://disruptor.googlecode.com/files/Disruptor-1.0.pdf (Page 5,
> last paragraph)
> >
> >
>
> --
> Manik Surtani
>
>
>
>
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