Date: Thu, 18 Nov 2004 13:00:38 +0000 (GMT) From: Robert Watson <rwatson@freebsd.org> To: Wilko Bulte <wb@freebie.xs4all.nl> Cc: freebsd-stable@freebsd.org Subject: Re: serious networking (em) performance (ggate and NFS) problem Message-ID: <Pine.NEB.3.96L.1041118125822.66045D-100000@fledge.watson.org> In-Reply-To: <20041118124902.GC75559@freebie.xs4all.nl>
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On Thu, 18 Nov 2004, Wilko Bulte wrote: > On Thu, Nov 18, 2004 at 12:27:44PM +0000, Robert Watson wrote.. > > > > On Wed, 17 Nov 2004, Emanuel Strobl wrote: > > > > > I really love 5.3 in many ways but here're some unbelievable transfer > > > rates, after I went out and bought a pair of Intel GigaBit Ethernet > > > Cards to solve my performance problem (*laugh*): > > > > I think the first thing you want to do is to try and determine whether the > > problem is a link layer problem, network layer problem, or application > > (file sharing) layer problem. Here's where I'd start looking: > > And you definitely want to look at polling(4) He did, but he set the HZ at 256, which is sufficiently low as to guarantee a substantial increase in latency, and likely guarantee interface and socket queue overruns (although I haven't done the math to verify that is the case). Between the very finite sizes of ifnet send queues, socket buffers, and if_em descriptors, and on-board buffers on the card, high latency polling can result in lots of packet loss and delay under load. Hence the recommendation of a relatively high value of HZ so that the queues in the driver are drained regularly, and sends acknowledged so that the sent mbufs can be reclaimed and reused. Robert N M Watson FreeBSD Core Team, TrustedBSD Projects robert@fledge.watson.org Principal Research Scientist, McAfee Research
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