Date: Tue, 8 Sep 2009 01:16:37 -0400 (EDT) From: Garrett Wollman <wollman@hergotha.csail.mit.edu> To: rysto32@gmail.com Cc: current@freebsd.org Subject: Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing) Message-ID: <200909080516.n885Gba0065213@hergotha.csail.mit.edu> In-Reply-To: <bc2d970909071917n12256d9ucee24327aec3d527@mail.gmail.com> References: <4A93AFF9.1060201@web.de> <52d4a3890908250321u746e5757u136030bcbc19208d@mail.gmail.com> <4A93BF0C.8040601@web.de> <20090826221001.GA1070@triton8.kn-bremen.de> <4A96C8D9.6070804@web.de> <20090829211848.GA59305@triton8.kn-bremen.de> <4A9B800F.1040209@web.de> <20090831212723.GA32448@triton8.kn-bremen.de> <20090901201248.GA60123@triton8.kn-bremen.de> <20090907205955.GA91866@triton8.kn-bremen.de>
next in thread | previous in thread | raw e-mail | index | archive | help
In article <bc2d970909071917n12256d9ucee24327aec3d527@mail.gmail.com>, Ryan Stone <rysto32@gmail.com> wrote: >I'm not entirely clear on why it's done this way, but the timer is run at >twice hz for statistics-gathering purposes*. CPU usage statistics gathering >is driven off of the timer interrupt. Running the timer at twice hz may be >an attempt to eliminate clock-aliasing problems; if so, it's a poor way of >doing so. The statistics timer is supposed to be jittered with an exponential distribution, so that applications cannot avoid being charged for CPU time by running synchronously (and out-of-phase) with the timer. This was historically broken on PC hardware, and is probably still broken on SMP PC hardware, because there are insufficient programmable timer interrupts. Ideally, you'd like a distinct statistics timers on each CPU, with a sufficiently (quickly) programmable period. -GAWollman
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200909080516.n885Gba0065213>