Re: [Xen-devel] [PATCH RFC 00/49] xen: add core scheduling support

[Rian Quinn <rianquinn@xxxxxxxxx>]

Makes sense. The reason I ask is we currently have to disable HT due
to L1TF until a scheduler change is made to address the issue and the
#1 question everyone asks is what will that do to performance so any
info on that topic and how a patch like this will address the L1TF
issue is most helpful.

On Fri, Mar 29, 2019 at 11:49 AM Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> wrote:
>
> On 29/03/2019 17:39, Rian Quinn wrote:
> > Out of curiosity, has there been any research done on whether or not
> > it makes more sense to just disable CPU threading with respect to
> > overall performance? In some of the testing that we did with OpenXT,
> > we noticed in some of our tests a performance increase when
> > hyperthreading was disabled. I would be curious what other research
> > has been done in this regard.
> >
> > Either way, if threading is enabled, grouping up threads makes a lot
> > of sense WRT some of the recent security issues that have come up with
> > Intel CPUs.
>
> There has been plenty of academic research done, and there are real
> usecases where disabling HT improves performance.
>
> However, there are plenty when it doesn't.  During L1TF testing,
> XenServer measured one typical usecase (agregate small packet IO
> throughput, which is representative of a load of webserver VMs) which
> took a 60% perf hit.
>
> 10% of this was the raw L1D_FLUSH hit, while 50% of it was actually due
> to the increased IO latency of halving the number of vcpus which could
> be run concurrently.
>
> As for core aware scheduling, even if nothing else, grouping things up
> will get you better cache sharing from the VM's point of view.
>
> As you can probably tell, the answer is far too workload dependent to
> come up with a general rule, but at least having the options available
> will let people experiment.
>
> ~Andrew

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