debug mode performance patch

[François Dumont]

On 11/19/2010 12:54 AM, Paolo Carlini wrote:
> Hi,
>
>    
>> I thought about suggesting that too. François's improved get_mutex
>> function only uses the lowest few bits, and I don't know how well
>> Hash_bytes works if you don't use the whole value ... probably still
>> better distributed than simply taking the address, and might avoid
>> pathological cases where distinct pairs of objects always get the same
>> two mutexes.
>>      
>    
>>      
> If there aren't other issues, I think the new Murmur hash is pretty good even when only a limited range of values is eventually computed. Matt told me that if only we could use it uniformly over *all* the std::hash specializations he would even feel comfortable with simplifying the unordered containers to use a power of two number of buckets. Anyway, in case if need, in the .cc file we also have a reference to the main Google web page.
>
> Paolo
Well, I consider this code as a temporary limited workaround to the 
contention issue so I made it simple. Ideally a mutex should be 
associated to each safe sequence. Do you think that this workaround is 
going to last for a long time ? Otherwise I do not see the point of 
spending too much time on the best method to associate the mutex to the 
sequence.

François