Date: Fri, 13 Aug 2004 13:28:51 -0400 (EDT) From: Andrew Gallatin <gallatin@cs.duke.edu> To: John Baldwin <jhb@FreeBSD.org> Cc: freebsd-current@FreeBSD.org Subject: Re: Is the TSC timecounter safe on SMP system? Message-ID: <16668.64083.212658.727644@grasshopper.cs.duke.edu> In-Reply-To: <200408131326.16412.jhb@FreeBSD.org> References: <16668.61707.474283.639200@grasshopper.cs.duke.edu> <200408131326.16412.jhb@FreeBSD.org>
next in thread | previous in thread | raw e-mail | index | archive | help
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? Drew
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?16668.64083.212658.727644>