netdev - Re: [PATCH 1/5] net: mvneta: increase the 64-bit rx/tx stats out of the hot path

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Date:	Mon, 13 Jan 2014 04:06:51 +0100
From:	Willy Tarreau <w@....eu>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Gregory CLEMENT <gregory.clement@...e-electrons.com>
Subject: Re: [PATCH 1/5] net: mvneta: increase the 64-bit rx/tx stats out of the hot path

On Sun, Jan 12, 2014 at 04:49:52PM -0800, Eric Dumazet wrote:
> On Sun, 2014-01-12 at 10:31 +0100, Willy Tarreau wrote:
> > Better count packets and bytes in the stack and on 32 bit then
> > accumulate them at the end for once. This saves two memory writes
> > and two memory barriers per packet. The incoming packet rate was
> > increased by 4.7% on the Openblocks AX3 thanks to this.
> > 
> > Cc: Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
> > Cc: Gregory CLEMENT <gregory.clement@...e-electrons.com>
> > Signed-off-by: Willy Tarreau <w@....eu>
> > ---
> >  drivers/net/ethernet/marvell/mvneta.c | 15 +++++++++++----
> >  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> 
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>
> 
> Note that with such a cost, one has to wonder why we keep 64bit stats
> for this NIC on 32bit hosts...

At least this avoids wrapping if stats are not retrieved often enough.
As someone who had to support 32-bit stats in production on a firewall
running on kernel 2.4, I can say it really becomes a problem to graph
activity if stats are not collected as often as every 30 seconds, which
is short in certain environments.

Thanks,
Willy

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