Date: Fri, 13 Aug 2004 10:38:29 -0700 (PDT) From: Doug White <dwhite@gumbysoft.com> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: freebsd-current@freebsd.org Subject: Re: Is the TSC timecounter safe on SMP system? Message-ID: <20040813103710.F93695@carver.gumbysoft.com> In-Reply-To: <16668.61707.474283.639200@grasshopper.cs.duke.edu> References: <16668.61707.474283.639200@grasshopper.cs.duke.edu>
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On Fri, 13 Aug 2004, 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? If you like your ticks bouncing around; you'll get different values depending on which CPU you read. > 2) If yes, would it be safe to switch to TSC if this was a real > SMP system with multiple physical cpus? TSC in a single-package HTT situation would be interesting to research, if it is shared across the pipelines. -- Doug White | FreeBSD: The Power to Serve dwhite@gumbysoft.com | www.FreeBSD.org
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