Re: [Xen-devel] [PATCH v9] new config option vtsc_tolerance_khz to avoid TSC emulation

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 01.10.18 at 14:39, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 07/06/18 14:08, 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.
> 
> Do you have any further details of the systems involved?  If they are
> identical systems, they should all have the same real TSC frequency, and
> its a known issue that Xen isn't very good at working out the
> frequency.  TBH, fixing that would be far better overall.

Are you convinced all parts match their nominal frequency without
_any_ deviation? If that was the case, we could indeed use CPUID
leaves 0x15 / 0x16 output, if available. But I very much doubt this.
As an example, here's what bare metal Linux says on my newest
system:

tsc: Detected 2600.000 MHz processor
tsc: Refined TSC clocksource calibration: 2591.990 MHz

Xen figures:

(XEN) Detected 2592.107 MHz processor.

And then after another re-boot bare metal Linux again

tsc: Refined TSC clocksource calibration: 2592.008 MHz

Jan



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