netdev - Re: [RFC PATCH 0/2] Coalesce MMIO writes for transmits

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Message-ID: <20120712102331.42a7b041@nehalam.linuxnetplumber.net>
Date:	Thu, 12 Jul 2012 10:23:31 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net,
	jeffrey.t.kirsher@...el.com, edumazet@...gle.com,
	bhutchings@...arflare.com, therbert@...gle.com,
	alexander.duyck@...il.com
Subject: Re: [RFC PATCH 0/2] Coalesce MMIO writes for transmits

On Wed, 11 Jul 2012 17:25:58 -0700
Alexander Duyck <alexander.h.duyck@...el.com> wrote:

> This patch set is meant to address recent issues I found with ixgbe
> performance being bound by Tx tail writes.  With these changes in place
> and the dispatch_limit set to 1 or more I see a significant increase in
> performance.
> 
> In the case of one of my systems I saw the routing rate for 7 queues jump
> from 10.5 to 11.7Mpps.  The overall increase I have seen on most systems is
> something on the order of about 15%.  In the case of pktgen I have also
> seen a noticeable increase as the previous limit for transmits was
> ~12.5Mpps, but with this patch set in place and the dispatch_limit enabled
> the value increases to ~14.2Mpps.
> 
> I expected there to be an increase in latency, however so far I have not
> ran into that.  I have tried running NPtcp tests for latency and seen no
> difference in the coalesced and non-coalesced transaction times.  I welcome
> any suggestions for tests I might run that might expose any latency issues
> as a result of this patch.
> 
> ---
> 
> Alexander Duyck (2):
>       ixgbe: Add functionality for delaying the MMIO write for Tx
>       net: Add new network device function to allow for MMIO batching
> 
> 
>  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c |   22 +++++++-
>  include/linux/netdevice.h                     |   57 +++++++++++++++++++++
>  net/core/dev.c                                |   67 +++++++++++++++++++++++++
>  net/core/net-sysfs.c                          |   36 +++++++++++++
>  4 files changed, 180 insertions(+), 2 deletions(-)
> 

This is a good idea. I was thinking of adding a multi-skb operation
to netdevice_ops to allow this. Something like ndo_start_xmit_pkts but
the problem is how to deal with the boundary case where there is only
a limited number of slots in the ring.  Using a "that's all folks"
operation seems better.
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