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Re: [Xen-devel] CAP and performance problem

On Tue, May 21, 2013 at 10:02 AM, Massimo Canonico <mex@xxxxxxxxxxxx> wrote:
> Hi,
> I sent the following problem on xen-user ML without an answer. I hope I'll
> get one in this ML.
> My application is written in std C++ and it makes a matrix multiplication:
> so it uses only CPU and memory (no I/O, no network).
> I'm quite surprise that with CAP = 100% I got my results in about 600
> seconds and with CAP = 50% I got my results in about 1800 seconds (around 3
> times longer).
> For this kind of application I was expecting to get results in about 1200
> seconds (2 times longer) for the second scenario with respect to the first
> one.
> Of course, the HW and SW are exactly the same for the 2 experiments.
> Am I wrong or the CAP mechanism is not working well?

How many vcpus do you have?

I'm not sure the CAP stuff actually does anything sensible with
multiple cpus -- e.g., a VM with 2 vcpus maxed out will typically
actually get 200%, so will a CAP force both of them to get only 25%

The other possibility if you've got multiple vcpus is that you're
running into something I call "concurrency hazard": if one vcpu grabs
a spinlock and then is paused in order to enforce the cap, and another
vcpu tries to grab that spinlock, it will just spin until the first
vcpu manages to wake up and release it again.


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