netdev - Re: [PATCH net-next] net: increase LL_MAX_HEADER if HYPERV

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Date:	Tue, 03 Nov 2015 10:33:19 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	kys@...rosoft.com
Cc:	eric.dumazet@...il.com, haiyangz@...rosoft.com,
	edumazet@...gle.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: increase LL_MAX_HEADER if HYPERV_NET is
 enabled

From: KY Srinivasan <kys@...rosoft.com>
Date: Tue, 3 Nov 2015 07:59:36 +0000

> I have implemented the scheme we had discussed a few weeks ago. In
> this new implementation our driver is NOT requesting addition
> headroom - rndis header and the per packet state is being maintained
> outside of the skb. What I am seeing is that when I have
> LL_MAX_HEADER set to 220 bytes, even though our driver is not using
> the additional head room, I see about a 10% boost in the peak
> performance (about 34 Gbps on a 40Gbps interface). However, when I
> set the LL_MAX_HEADER value to the current default, the peak
> performance drops back to what we currently have (around 31
> Gbps). In both these cases, there is no reallocation of skb since no
> additional headroom is being requested and yet there is a
> significant difference in performance.  I trying to figure out why
> this is the case, your insights will be greatly appreciated.

It probably has something to do with cache line or data alignment.
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