[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: [Xen-devel] Time skew on HP DL785 (and possibly other boxes)
>From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx] >Sent: 2009年4月5日 15:56 > >One concern I have however, is Intel's >X86_FEATURE_CONSTANT_TSC logic. This >was added by them to prevent TSCs from diverging due to Cx deep sleep >states, by observing that usually all TSCs will tick at the >same exact rate, Here one correction is, that constant tsc logic is introduced for P-states instead of C-states, to have TSC always stepping in constant pace on a given processor, regardless of whatever opertion point is being requested by cpufreq governor. It doesn't say anything that all TSCs tick at same rate however. >so all that needs to be done is to rewrite all AP TSCs to that >of the BP >periodically. This seems to work well on small systems, but >the trigger for >this mode is rather suspicious. CONSTANT_TSC feature means >that a CPU's TSC >is invariant across frequency/voltage changes -- it *doesn't* >mean that all >TSCs across a large MP box are at matched frequency! I wonder You're exactly right here. To use it does require that all cpus are driven by a single crystal, which is not true for a large system with multipe crystals. So this approach (sync all TSCs to minimize skews caused by TSC stop from deep C-states) doesn't work in all cases. >whether this >optimisation will bite us on big iron? Probably it ought to >disable itself >if it detects significant TSC divergence, or at the very least maybe we >should add a command-line option to disable (or enable?) it. I guess thing won't be that worse in this C-state specific area. Large system based on Intel core-i7 processors or later always have invariant tsc feature (non-stop tsc) integrated, and thus no software recovery is required, while in the meantime, iirc, previous large scale servers don't have deep C-state (>=C3) implemented and so it's not an issue too. :-) Thanks, Kevin _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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