netdev - Re: [PATCH net-next] r8169: use unlimited DMA burst for TX

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Date:	Mon, 10 Sep 2012 15:50:46 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	mschmidt@...hat.com
Cc:	netdev@...r.kernel.org, romieu@...zoreil.com,
	hayeswang@...ltek.com, nic_swsd@...ltek.com, ivecera@...hat.com
Subject: Re: [PATCH net-next] r8169: use unlimited DMA burst for TX

From: Michal Schmidt <mschmidt@...hat.com>
Date: Mon, 10 Sep 2012 01:55:26 +0200

> The r8169 driver currently limits the DMA burst for TX to 1024 bytes. I have
> a box where this prevents the interface from using the gigabit line to its full
> potential. This patch solves the problem by setting TX_DMA_BURST to unlimited.
> 
> The box has an ASRock B75M motherboard with on-board RTL8168evl/8111evl
> (XID 0c900880). TSO is enabled.
> 
> I used netperf (TCP_STREAM test) to measure the dependency of TX throughput
> on MTU. I did it for three different values of TX_DMA_BURST ('5'=512, '6'=1024,
> '7'=unlimited). This chart shows the results:
> http://michich.fedorapeople.org/r8169/r8169-effects-of-TX_DMA_BURST.png
> 
> Interesting points:
>  - With the current DMA burst limit (1024):
>    - at the default MTU=1500 I get only 842 Mbit/s.
>    - when going from small MTU, the performance rises monotonically with
>      increasing MTU only up to a peak at MTU=1076 (908 MBit/s). Then there's
>      a sudden drop to 762 MBit/s from which the throughput rises monotonically
>      again with further MTU increases.
>  - With a smaller DMA burst limit (512):
>    - there's a similar peak at MTU=1076 and another one at MTU=564.
>  - With unlimited DMA burst:
>    - at the default MTU=1500 I get nice 940 Mbit/s.
>    - the throughput rises monotonically with increasing MTU with no strange
>      peaks.
> 
> Notice that the peaks occur at MTU sizes that are multiples of the DMA burst
> limit plus 52. Why 52? Because:
>   20 (IP header) + 20 (TCP header) + 12 (TCP options) = 52
> 
> The Realtek-provided r8168 driver (v8.032.00) uses unlimited TX DMA burst too,
> except for CFG_METHOD_1 where the TX DMA burst is set to 512 bytes.
> CFG_METHOD_1 appears to be the oldest MAC version of "RTL8168B/8111B",
> i.e. RTL_GIGA_MAC_VER_11 in r8169. Not sure if this MAC version really needs
> the smaller burst limit, or if any other versions have similar requirements.
> 
> Signed-off-by: Michal Schmidt <mschmidt@...hat.com>

I really need Francois et al. to take a look at this before even thinking
about applying it.
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