Date: Mon, 12 Jun 2006 21:02:51 +0100 (BST) From: Robert Watson <rwatson@FreeBSD.org> To: Danial Thom <danial_thom@yahoo.com> Cc: freebsd-performance@freebsd.org Subject: Re: Initial 6.1 questions Message-ID: <20060612210054.S26068@fledge.watson.org> In-Reply-To: <20060612195754.72452.qmail@web33306.mail.mud.yahoo.com> References: <20060612195754.72452.qmail@web33306.mail.mud.yahoo.com>
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On Mon, 12 Jun 2006, Danial Thom wrote: >> This is a design change that is in the process of being reconsidered. I >> expect that HZ will not be 1000 in 7.x, but can't tell you whether it will >> go back to 100, or some middle ground. There are a number of benefits to a >> higher HZ, not least is more accurate timing of some network timer events. >> Since I don't have my hands in the timer code, I can't speak to what the >> decision process here is, or when any change might happen, but I do expect >> to see some change. > > Will anything break if I tweek this downward? No, shouldn't do. I wouldn't go below 100 though, as things like process statistics, involuntary context switches, etc, are all affected. >> Finally, there is a known performance problem involving loopback network >> traffic and preemption, which results in additional context switches. You >> may want to try disabling preemption and see if/how that impacts your >> numbers. There has been seen quite a bit of discussion of this problem, and >> I expect to see a solution for it in the near future. This problem does >> not manifest for remote traffic, only loopback traffic. > > I'm sending this traffic from an external device, receiving on an em > controller with blackhole set to 1. So I assume this loopback issue doesn't > apply to this test? The above comments only refer to traffic being sent over if_loop interfaces or certain other deferred work scenarios. Basically, defering of work to the netisr from a user thread rather than an interrupt thread results in a premature context switch. Robert N M Watson Computer Laboratory Universty of Cambridge
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