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Date:	Tue, 27 Oct 2009 07:07:31 +0100
From:	Eric Dumazet <>
To:	Stephen Hemminger <>
CC:	Andrew Morton <>,
	Linus Torvalds <>,
	Octavian Purdila <>,,,
	Al Viro <>
Subject: Re: [PATCH] dcache: better name hash function

Stephen Hemminger a écrit :
> 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;

        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

        movl    $211, %eax
        mull    4(%esp)
        movl    %edx, %eax

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