On 03/28/2018 05:52 PM, Wei Liu wrote: > On Wed, Mar 28, 2018 at 05:49:28PM +0100, George Dunlap wrote: >> 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. >> > > I have a rough idea how the code should look like -- the patch in its > current form looks mostly OK, but not sure if this approach in general > is future proof.
What do you mean? And do you have an idea for a better way to solve the problem? Or do you think it's not a very big problem? -George -George _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
