netdev - Re: net: more accurate skb truesize

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Date:	Tue, 10 Apr 2012 10:36:57 +0200
From:	Michal Simek <monstr@...str.eu>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	netdev@...r.kernel.org,
	John Williams <john.williams@...alogix.com>,
	David Miller <davem@...emloft.net>
Subject: Re: net: more accurate skb truesize - regression on Microblaze

On 04/10/2012 10:27 AM, Eric Dumazet wrote:
> On Tue, 2012-04-10 at 10:10 +0200, Michal Simek wrote:
>> Hi Eric,
>>
>> I have spent some time to investigate what it is causing regression on our network bechmark
>> and I have identified that your patch "net: more accurate skb truesize" is causing that.
>>
>> I see regression especially on incoming tcp (RX) connection which is 20%. On TX I see regression till 5%.
>>
>> In general microblaze systems are pretty sensitive for memory usage and working with it.
>> Increasing packet sizes has big impact on performance.
>> I was surprised that regression is so high.
>>
>>   From microblaze point of view is more important to remove that performance regression
>> than having more accurate memory accounting. 20% performance regression is simple so high
>> for that.
>> Isn't there any other way to doing better memory accounting?
>> Or is there any workaround which would be possible to use?
>>
>> Thanks,
>> Michal
>>
>
> Hi Michal
>
> We are currently working on the issue.
>
> TCP stack was a bit optimistic in the ideal skb->len / skb->truesize
> ratio that happened to 'mostly work' before my patch, but not anymore
> after it.
>
> memory accounting was wrong, we really wanted to be accurate, or risk
> OOM and crashes.
>
> Now we must fix tcp.
>
> In the meantime you could try :
>
> echo 1>/proc/sys/net/ipv4/tcp_adv_win_scale

will retest it and let you know.

Thanks,
Michal



-- 
Michal Simek, Ing. (M.Eng)
w: www.monstr.eu p: +42-0-721842854
Maintainer of Linux kernel 2.6 Microblaze Linux - http://www.monstr.eu/fdt/
Microblaze U-BOOT custodian
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