Date: Tue, 13 Jun 2006 20:01:40 +0100 (BST) From: Robert Watson <rwatson@FreeBSD.org> To: Danial Thom <danial_thom@yahoo.com> Cc: freebsd-performance@freebsd.org, David Xu <davidxu@freebsd.org> Subject: Re: Initial 6.1 questions Message-ID: <20060613195113.T26068@fledge.watson.org> In-Reply-To: <20060613184336.60229.qmail@web33314.mail.mud.yahoo.com> References: <20060613184336.60229.qmail@web33314.mail.mud.yahoo.com>
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On Tue, 13 Jun 2006, Danial Thom wrote: >> Two types of measurements are taken: sampled ticks regarding whether the >> system as a while is in {user, nice, system, intr, idle}, and then sampling >> for individual processes. Right now, the system measurements are kept in a >> simple array of tick counters called cp_time. John Baldwin and others have >> changes that make these tick counters per-CPU. The lines at the top of >> top(1)'s output are derived from those tick counters. Ticks are measured >> on each CPU, so those are a summary across all CPUs. To add cpustat >> support, we need to merge John's patch to make cp_time per-CPU (ie., >> different counters for different CPUs) and teach the userland tools to >> retrieve them. When you run top you'll notice that it adjusts the >> measurements each refresh. In effect, what it's doing is sampling the >> change in tick counts over the window, pulling down the new values and >> calculating the percentages of ticks in each "bucket" in the last window. > > That doesn't explain why the Top line shows 99.6% idle, but the cpu idle > threads are showing significant usage. > > I'm getting a constant 6000 Interrupts / Second on my em controller, yet top > jumps all over the place; sitting at 99% idle for 10 seconds, then jumping > to 50%, then somewhere in between. It seems completely unreliable. The load > I'm applying is constant. I can't speak to the details of the thread/process use sampling model. Top uses something called the "weighted cpu percentage" by default; you can switch to "unweighted" using the -C argument. The top documentation fails to document the semantics of the percentages, but I suspect -C will give you more of what you expect. The weighted CPU measurement takes into account process history, so it takes a while for sudden spike in CPU use to be fully reflected, and you may see seemingly counter-intuitive results, such as the appearance of greater than 100% CPU use. Try out -C and see if you see something that makes more sense? Robert N M Watson Computer Laboratory Universty of Cambridge
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