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Re: [Xen-devel] [PATCH RFC 00/49] xen: add core scheduling support
- To: Rian Quinn <rianquinn@xxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 29 Mar 2019 17:48:59 +0000
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- Cc: Kevin Tian <kevin.tian@xxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Jun Nakajima <jun.nakajima@xxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Robert VanVossen <Robert.VanVossen@xxxxxxxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Paul Durrant <paul.durrant@xxxxxxxxxx>, Joshua Whitehead <josh.whitehead@xxxxxxxxxxxxxxx>, Meng Xu <mengxu@xxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Fri, 29 Mar 2019 17:49:10 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
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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