linux-kernel - Re: loaded router, excessive getnstimeofday in oprofile

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Date:	Wed, 27 Aug 2008 09:57:54 -0700
From:	Rick Jones <rick.jones2@...com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Evgeniy Polyakov <johnpol@....mipt.ru>,
	Eric Dumazet <dada1@...mosbay.com>,
	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 wrote:
> 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.

Ah, since that part of the discussion wasn't in the quoted text I 
assumed you were talking about the disabling of interrupt coalescing.

rick jones

> 
> 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.
> 
> -Andi

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