netdev - Re: [PATCH] Software receive packet steering

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Message-Id: <20090420.034628.224469114.davem@davemloft.net>
Date:	Mon, 20 Apr 2009 03:46:28 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	andi@...stfloor.org
Cc:	therbert@...gle.com, netdev@...r.kernel.org
Subject: Re: [PATCH] Software receive packet steering

From: Andi Kleen <andi@...stfloor.org>
Date: Mon, 20 Apr 2009 12:32:29 +0200

> Tom Herbert <therbert@...gle.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
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