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.