-----Original Message-----
From: David Miller [mailto:davem@davemloft.net]
Sent: Tuesday, November 3, 2015 7:33 AM
To: KY Srinivasan <kys@microsoft.com>
Cc: eric.dumazet@gmail.com; Haiyang Zhang <haiyangz@microsoft.com>;
edumazet@google.com; netdev@vger.kernel.org
Subject: Re: [PATCH net-next] net: increase LL_MAX_HEADER if HYPERV_NET
is enabled

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

quoted

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