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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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.

Wei.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel