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Re: [Xen-users] Give dom0 2 pinned vcpus, but share one with domU

Not sure this is the correct answer, but how about NOT using the dom0_max_vcpus and dom0_vcpus_pin options. I've done some tests with running heavy domU and dom0 loads and without using CPU pinning and max_vcpus the dom0 would take whatever CPU resources it could get. In my test case I used a Windows 7 domU running Lightroom raw photo conversion to jpegs while using handbrake to transcode videos under dom0. My CPU has 12 vcpus (6-core Intel), and I gave domU 10 vcpus. Whenever domU didn't use its allocated VCPUs the dom0 would be able to utilise them.
There is an option to adjust the credit scheduler  - see http://wiki.xen.org/wiki/Credit_Scheduler. More on Xen tuning can be found http://wiki.xenproject.org/wiki/Tuning_Xen_for_Performance.
On Monday, May 19, 2014 8:00 PM, jumperalex <alex@xxxxxxxxxxxxxx> wrote:

Is it possible to give dom0 two vcpus, pinned, but also allow one of those to
be shared with a domU?

So right now I'm setup like this:
dom0: dom0_max_vcpus=1 dom0_vcpus_pin
archVM: vcpu="1-7"

In most of my use cases this works fine, but in a few (rsync specifically)
my dom0 is just getting hammered and it is slowing a local rsync between
domU data.img and dom0 raid array.

Someone else suggested turning off the md5 check in rsync but I'm not
thrilled about turning off hashing of a backup and I want to see if I can
solve the problem with good provisioning first.

So what if I did this:
dom0: dom0_max_vcpus=*2* dom0_vcpus_pin
archVM: vcpu = "1-7"

So I'm giving dom0 2 vcpus which will be pinned to cores 0,1 but I'm still
allowing archVM access to core 1 but not core 0

Will this even work?

How will Xen deal with dividing up core1's cycles?

If there is contention for core1 will that cause a lot of context switching
between cores 0 and 1 within dom0?  Or will core 1 just be shared ~50/50
while core 0 does dom0's heavy lifting?

I know I could just give archVM 6 cores instead of 7, but I really want it
to have access to as many cores as possible for heavy transcoding loads but
I also know I need to dedicate one to dom0 so this is my compromise

View this message in context: http://xen.1045712.n5.nabble.com/Give-dom0-2-pinned-vcpus-but-share-one-with-domU-tp5722792.html
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