netdev - Re: loaded router, excessive getnstimeofday in oprofile

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Message-ID: <48B58E6B.1030302@cosmosbay.com>
Date:	Wed, 27 Aug 2008 19:27:07 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Rick Jones <rick.jones2@...com>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	Denys Fedoryshchenko <denys@...p.net.lb>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: loaded router, excessive getnstimeofday in oprofile

Andi Kleen a écrit :
> On Wed, Aug 27, 2008 at 09:49:10AM -0700, Rick Jones wrote:
>> Andi Kleen wrote:
>>>> Those banks really want to crank down on latency - to the point they 
>>>> start disabling interrupt coalescing.  I bet they'd toss anything out 
>>>> they could to shave another microsecond.
>>>
>>> This change would actually likely lower their latency.
>> I'm guessing you mean increase their latency?  I agree, it could - 
>> depends entirely on the PPS in production I suspect.
> 
> No, moving the time stamps into the socket decreases latency
> for all packets that don't need time stamps. And they likely 
> have some packets which don't need time stamps too.
> 
> As a secondary effect if they use a RT kernel it might
> be also beneficial to do the (depending on the platform)
> costly time stamp in the lower priority socket context
> than in the high priority interrupt thread.
>

Doing the expensive timestamping in a possibly delayed thread (ie some milliseconds
 after hardware notification) is wrong/useless.

Better use plain xtime instead of getnstimeofday() in this case.

We could provide a sysctl setting so that admin can chose between precise timestamps
(current behavior) or fast but low resolution timestamping (xtime based)





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