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

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Date:	Tue, 3 Nov 2015 18:09:52 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	David Miller <davem@...emloft.net>
CC:	"eric.dumazet@...il.com" <eric.dumazet@...il.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH net-next] net: increase LL_MAX_HEADER if HYPERV_NET is
 enabled



> -----Original Message-----
> From: David Miller [mailto:davem@...emloft.net]
> Sent: Tuesday, November 3, 2015 7:33 AM
> To: KY Srinivasan <kys@...rosoft.com>
> Cc: eric.dumazet@...il.com; Haiyang Zhang <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.

Thanks David. I too am leaning towards the same conclusion. Since I am
not using any part of skb, it looks like the alignment issue is solved by
having a larger LL_MAX_HEADER (220 bytes to be specific). I will do some
bare metal testing on the setup we have and report back.

Regards,

K. Y 
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