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From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH] Software receive packet steering
Date: Mon, 20 Apr 2009 03:46:28 -0700 (PDT)
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From: Andi Kleen <andi@firstfloor.org>
Date: Mon, 20 Apr 2009 12:32:29 +0200

> Tom Herbert <therbert@google.com> writes:
> 
>> +got_hash:
>> +	hash %= cpus_weight_nr(mask);
> 
> That looks rather heavyweight even on modern CPUs. I bet it's 40-50+ cycles
> alone forth the hweight and the division. Surely that can be done better?

The standard way to do this is to compute a 32-bit jenkins
hash, and do a 64-bit multiply of this value with a suitable
reciprocol.

This is what skb_tx_hash() is doing in net/core/dev.c