netdev - Re: [IPoIB] Identify multicast packets and fix IGMP breakage V3

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Date:	Thu, 26 Aug 2010 17:07:16 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	cl@...ux.com
Cc:	jgunthorpe@...idianresearch.com, rdreier@...co.com,
	linux-rdma@...r.kernel.org, ogerlitz@...taire.com,
	yosefe@...taire.com, netdev@...r.kernel.org
Subject: Re: [IPoIB] Identify multicast packets and fix IGMP breakage V3

From: Christoph Lameter <cl@...ux.com>
Date: Thu, 26 Aug 2010 18:57:09 -0500 (CDT)

> On Thu, 26 Aug 2010, Jason Gunthorpe wrote:
> 
>> I think doing the memcmp only in the multicast path should be
>> reasonable overhead wise.
> 
> Thats is not always possible. Here the multicast path is the
> default path that is taken 99% of the time.

The highest cost is bringing in that packet header's cache line, which
you've already done by reading the byte and checking for 0xff.

I doubt the memcmp() can possibly matter.
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