Message-ID: <4D290450.8070700@openwrt.org>
Date: Sat, 08 Jan 2011 17:41:52 -0700
From: Felix Fietkau <nbd@openwrt.org>
MIME-Version: 1.0
To: Ben Greear <greearb@candelatech.com>
CC: linux-wireless@vger.kernel.org, ath9k-devel@venema.h4ckr.net
Subject: Re: [PATCH] ath9k:  Implement rx copy-break.
References: <1294500800-29191-1-git-send-email-greearb@candelatech.com> <4D28FF57.9040706@openwrt.org> <4D290307.4080807@candelatech.com>
In-Reply-To: <4D290307.4080807@candelatech.com>
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Sender: linux-wireless-owner@vger.kernel.org

On 2011-01-08 5:36 PM, Ben Greear wrote:
> On 01/08/2011 04:20 PM, Felix Fietkau wrote:
>>  I think this should be dependent on packet size, maybe even based on the architecture. Especially on embedded hardware, copying large frames is probably quite a
>>  bit more expensive than allocating large buffers. Cache sizes are small, memory access takes several cycles, especially during concurrent DMA.
>>  Once I'm back home, I could try a few packet size threshold to find a sweet spot for the typical MIPS hardware that I'm playing with. I expect a visible
>>  performance regression from this patch when applied as-is.
>
> I see a serious performance improvement with this patch.  My current test is sending 1024 byte UDP
> payloads to/from each of 60 stations at 128kbps.  Please do try it out on your system and see how
> it performs there.  I'm guessing that any time you have more than 1 VIF this will be a good
> improvement since mac80211 does skb_copy (and you would typically be copying a much smaller
> packet with this patch).
>
> If we do see performance differences on different platforms, this could perhaps be
> something we could tune at run-time.
What kind of system are you testing on? If it's a PC, then the 
performance characteristics will be completely different compared to 
embedded hardware. I've had to remove a few copybreak-like 
implementations from various ethernet drivers on similar hardware, 
because even taking the hit of unaligned load/store exceptions (which 
are already *very* expensive on MIPS) was less than copying the full 
packet data, even with packet sizes less than what you're using.

I don't have suitable test hardware with me right now, but I'll do some 
tests in a week or so.

- Felix