netdev - Re: [PATCH 07/15] ehea: Allocate large enough skbs to avoid partial cacheline DMA writes

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Date:	Thu, 12 May 2011 03:52:48 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Anton Blanchard <anton@...ba.org>
Cc:	leitao@...ux.vnet.ibm.com, michael@...erman.id.au,
	jesse@...nel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 07/15] ehea: Allocate large enough skbs to avoid
 partial cacheline DMA writes

On Thu, 2011-05-12 at 10:52 +1000, Anton Blanchard wrote:
> plain text document attachment (ehea_8.patch)
> The ehea adapter has a mode where it will avoid partial cacheline DMA
> writes on receive by always padding packets to fall on a cacheline
> boundary.
> 
> Unfortunately we currently aren't allocating enough space for a full
> ethernet MTU packet to be rounded up, so this optimisation doesn't hit.
> 
> It's unfortunate that the next largest packet size exposed by the
> hypervisor interface is 2kB, meaning our skb allocation comes out of a
> 4kB SLAB. However the performance increase due to this optimisation is
> quite large and my TCP stream numbers increase from 900MB to 1000MB/sec.

You can allocate page buffers and then split them into exactly 2K
segments.  If you switch from inet_lro to GRO then GRO will handle the
eventual skb allocation for you (even for non-mergeable packets).

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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