Date: Tue, 18 Oct 2005 11:01:02 -0400 From: John Baldwin <jhb@freebsd.org> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: cvs-src@freebsd.org, src-committers@freebsd.org, David Xu <davidxu@freebsd.org>, cvs-all@freebsd.org Subject: Re: cvs commit: src/sys/amd64/amd64 cpu_switch.S machdep.c Message-ID: <200510181101.03956.jhb@freebsd.org> In-Reply-To: <20051018094402.A29138@grasshopper.cs.duke.edu> References: <200510172310.j9HNAVPL013057@repoman.freebsd.org> <20051018094402.A29138@grasshopper.cs.duke.edu>
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On Tuesday 18 October 2005 09:44 am, Andrew Gallatin wrote: > David Xu [davidxu@FreeBSD.org] wrote: > > davidxu 2005-10-17 23:10:31 UTC > > > > FreeBSD src repository > > > > Modified files: > > sys/amd64/amd64 cpu_switch.S machdep.c > > Log: > > Micro optimization for context switch. Eliminate code for saving > > gs.base and fs.base. We always update pcb.pcb_gsbase and pcb.pcb_fsbase > > when user wants to set them, in context switch routine, we only need to > > write them into registers, we never have to read them out from registers > > when thread is switched away. Since rdmsr is a serialization instruction, > > micro benchmark shows it is worthy to do. > > Nice. This reduces lmbench context switch latency by about 0.4us (7.2 > -> 6.8us), and reduces TCP loopback latency by about 0.9us (36.1 -> > 35.2) on my dual core 3800+ > > It is a shame we can't find a way to use the TSC as a timecounter on > SMP systems. It seems that about 40% of the context switch time is > spent just waiting for the PIO read of the ACPI-fast or i8254 to > return. You can try it by just setting the kern.timecounter.smp_tsc=1 tunable on boot. -- John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve" = http://www.FreeBSD.org
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