On 03/28/2018 05:27 PM, Wei Liu wrote: > On Tue, Mar 27, 2018 at 11:26:55AM +0200, Olaf Hering wrote: >> Add an option to control when vTSC emulation will be activated for a >> domU with tsc_mode=default. Without such option each TSC access from >> domU will be emulated, which causes a significant perfomance drop for >> workloads that make use of rdtsc. >> >> One option to avoid the TSC option is to run domUs with tsc_mode=native. >> This has the drawback that migrating a domU from a "2.3GHz" class host >> to a "2.4GHz" class host may change the rate at wich the TSC counter >> increases, the domU may not be prepared for that. >> >> With the new option the host admin can decide how a domU should behave >> when it is migrated across systems of the same class. Since there is >> always some jitter when Xen calibrates the cpu_khz value, all hosts of >> the same class will most likely have slightly different values. As a >> result vTSC emulation is unavoidable. Data collected during the incident >> which triggered this change showed a jitter of up to 200 KHz across >> systems of the same class. >> >> Existing padding fields are reused to store vtsc_khz_tolerance as u16. >> > [...] >> index 2c1a6e1422..0b36265e4f 100644 >> --- a/docs/man/xl.cfg.pod.5.in >> +++ b/docs/man/xl.cfg.pod.5.in >> @@ -1891,6 +1891,16 @@ determined in a similar way to that of B<default> TSC >> mode. >> >> Please see B<xen-tscmode(7)> for more information on this option. >> >> +=item B<vtsc_tolerance_khz="KHZ"> >> + >> +B<(x86 only, relevant only for tsc_mode=default)> >> +When a domU is started, the CPU frequency of the host is used by the domU >> for >> +TSC related time measurement. Once the domU is either migrated or >> +saved/restored on another host that CPU frequency has to be emulated to >> avoid >> +timedrift. To avoid the performance penalty of the TSC emulation, allow a >> +certain amount of jitter of the measured CPU frequency on the hosts the domU >> +is supposed to run on. > > "Default value is 0, i.e. no tolerance". > > Can we get an agreement on whether this idea the right approach in > general before I do detail review?
I'm not super-familiar with this area, but looking from the outside I think Olaf's approach (having a "tolerance" for equivalence) makes sense. Can't comment on what kind of a hypercall it should be. -George _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
