netdev - Re: [PATCH] ibmveth: Increase default copybreak limits to 2k

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Date:	Thu, 21 Oct 2010 07:45:25 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	rcj@...ux.vnet.ibm.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] ibmveth: Increase default copybreak limits to 2k

From: Robert Jennings <rcj@...ux.vnet.ibm.com>
Date: Thu, 21 Oct 2010 09:37:53 -0500

> Increase the copybreak limits for rx and tx from 128 bytes
> to 2048 bytes. These limits were added by commits
> 8d86c61ae41d9068fd5e5cc01a4abd53c4fe3ab5 and
> c08cc3ccebd46dce44d13a8ce81d249e687eeb8a to make use of a
> bounce buffer for packets below 128 bytes.  This avoids
> tearing down and creating a TCE entry.
> 
> Performance testing shows that this default limit can be
> increased from 128 to 2048 for both rx and tx copybreak.
> This resulted in ~10% throughput increase for for packets
> that fit this limit without affecting performance for larger
> packets.
> 
> Signed-off-by: Robert Jennings <rcj@...ux.vnet.ibm.com>

For a default MTU of 1500 this means the copybreak will
never be used.

This makes absolutely no sense, are you doing these tests
with Jumbo frames?
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