[infinispan-dev] Equivalence vs. MurmurHash3

[Radim Vansa]

On 11/23/2015 01:07 PM, Sanne Grinovero wrote:
> +1
>
> See also https://issues.jboss.org/browse/ISPN-3905 ; although I was
> mostly concerned on it allocating on a (very) hot path and didn't look
> at it in terms of compatibility modes.
Yes, due to compatibility we cannot remove the UTF-8 encoding from 
MurmurHash3 since compatibility with clients (in other languages) 
depends on this as well, though, we could theoretically merge encoding 
and hashing into one function - UTF-8 encoder implementation looks quite 
simple (60 lines of code?) - could be worth it even if used only for 
server. However, my proposal was to remove that computation from 
hot-code path completely.

>
> Rather than xorring with "magic numbers" don't you think we
> Equivalence implementation should be able to rule on that?

We shouldn't require user to provide a pair of hashCode functions, I 
don't think that would work well in practice. Though, we could make the 
second function Java 8-default method (with return hashCode() ^ 
0xWHATEVER), still allowing it to be overridable.

Radim

>
> On 23 November 2015 at 10:26, Radim Vansa <rvansa at redhat.com> wrote:
>> Hi guys,
>>
>> I have noticed that even in library mode we use MurmurHash3 to find out
>> the segment for particular key. For strings, this involves encoding into
>> UTF-8 and computation of hashCode, instead of just reading the cached
>> value in string. Common objects just remix the bits of hashCode. When
>> user provides custom Equivalence with non-default hashCode, it is not
>> used to determine the segment.
>>
>> I think that in library mode we should rather use Equivalence.hashCode,
>> maybe XORed with some magic number so that there are less collisions in
>> DataContainer.
>>
>> If we simply replaced the function in CH, we would break the case when
>> user starts HR server on top of library mode, as the clients expect key
>> location based on MurmurHash3. ATM user only has to set
>> AnyServerEquivalence for keys in DC; we would need to detect
>> configuration with server equivalence and set CH function to MH3, and
>> probably also log some warning if the equivalence is set to unknown
>> class and CH function is not specified.
>>
>> WDYT?
>>
>> Radim
>>
>> --
>> Radim Vansa <rvansa at redhat.com>
>> JBoss Performance Team
>>
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-- 
Radim Vansa <rvansa at redhat.com>
JBoss Performance Team