Re: [Xen-devel] [PATCH v5] new config option vtsc_tolerance_khz to avoid TSC emulation

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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?

Wei.

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