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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) -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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