Date: Fri, 13 Aug 2004 13:38:37 -0400 From: John Baldwin <jhb@FreeBSD.org> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: freebsd-current@FreeBSD.org Subject: Re: Is the TSC timecounter safe on SMP system? Message-ID: <200408131338.37038.jhb@FreeBSD.org> In-Reply-To: <16668.64083.212658.727644@grasshopper.cs.duke.edu> References: <16668.61707.474283.639200@grasshopper.cs.duke.edu> <200408131326.16412.jhb@FreeBSD.org> <16668.64083.212658.727644@grasshopper.cs.duke.edu>
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On Friday 13 August 2004 01:28 pm, Andrew Gallatin wrote: > John Baldwin writes: > > On Friday 13 August 2004 12:49 pm, Andrew Gallatin wrote: > > > I have a system where the TSC timecounter is quite a bit more accurate > > > (or perhaps its just much cheaper) than the ACPI timecounter. This is > > > a single CPU, HTT system running an SMP kernel. > > > > > > A simple program which calls gettimeofday() in a tight loop, looking > > > for the microseconds to change sees ~998,000 microsecond updates/sec > > > with kern.timecounter.hardware=TSC, and 28,500 updates/sec with > > > ACPI-safe. > > > > > > 1) Is it safe to switch to TSC? > > > > > > 2) If yes, would it be safe to switch to TSC if this was a real > > > SMP system with multiple physical cpus? > > > > Probably not. The problem is that the TSC is not necessarily in sync > > between the CPUs so time would "jump around" as you migrated between > > CPUs. If you can get the TSC's synchronized between the CPUs and keep > > them that way then you can use the TSC (Linux does this FWIW). > > But on a single CPU HTT machine, does each HTT core reads the same TSC? I think they each have their own TSC. -- 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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