Re: [PATCH] dcache: better name hash function
From: Eric Dumazet
Date: Tue Oct 27 2009 - 02:08:31 EST
Stephen Hemminger a Ãcrit :
> One of the root causes of slowness in network usage
> was my original choice of power of 2 for hash size, to avoid
> a mod operation. It turns out if size is not a power of 2
> the original algorithm works fairly well.
Interesting, but I suspect all users have power of 2 tables :(
>
> On slow cpu; with 10million entries and 211 hash size
>
>
>
> How important is saving the one division, versus getting better
> distribution.
unsigned int fold1(unsigned hash)
{
return hash % 211;
}
Compiler uses a reciprocal divide because of 211 being a constant.
And you also could try following that contains one multiply only,
and check if hash distribution properties are still OK
unsigned int fold2(unsigned hash)
{
return ((unsigned long long)hash * 211) >> 32;
}
fold1:
movl 4(%esp), %ecx
movl $-1689489505, %edx
movl %ecx, %eax
mull %edx
shrl $7, %edx
imull $211, %edx, %edx
subl %edx, %ecx
movl %ecx, %eax
ret
fold2:
movl $211, %eax
mull 4(%esp)
movl %edx, %eax
ret
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