Hey Gabriel,
Remember that the goal of the scheduler isn't to enforce strict
equality of cpu time, but to divide cpu time according to the weight
while maximizing physical cpu usage (and thus total system
throughput). After a VM has used its allocated cpu time, it can still
get "spare" cpu cycles in a "best-effort" manner, if no VMs with
allocated cpu time left are currently running. This "best-effort" is
divided equally among vcpus, so a domain with more vcpus will
naturally get more of this "extra" time than a domain with less.
If I recall correctly, the SPECCPU suite uses real workloads, such as
bzip, gcc, and others. A lot of these workloads also include disk
I/O, which may cause vcpus to block. Blocking and waking of different
vcpus and VMs is bound to cause some interesting interactions between
VMs; for example, if a 1-vcpu and an 8-vcpu VM are running, and the
1-vcpu VM blocks, the 8-vcpu VM can use the extra processor time the
1-vcpu VM isn't using; however, if some of the 8-vcpu VM's vcpus
block, the 1-vcpu VM can't use the extra cpu time; the cpus just sit
idle.
If you want a "clean" scheduler test, you should instead run "while(1)
;" loops, which will never block, and will always consume all cpu time
available. My guess is if you do that, then the cpu time given to
each domain will be exactly according to their weight. On the other
hand, if you do a "kernbench" test, which will include a lot of
blocking, I suspect you may get even more disparity between the
runtimes.
-George
On Tue, Jul 22, 2008 at 4:43 AM, Gabriel Southern <gsouther@xxxxxxx> wrote:
> Hi Mark,
>
> Thanks for the reply, I'll be interested to see if you have any
> additional thoughts after I describe one of the tests that I have run.
>
> The system that I have been working with is a dual quad-core system so
> it has eight logical processors. Most of the tests that I have run
> have been with 8 VMs executing simultaneously with various different
> numbers of VCPUs for each VM. Most of the tests have been run with
> various benchmarks from the SPEC CPU2006 suite.
>
> One test that does not use the SPEC benchmarks and is probably the
> easiest to replicate is as follows:
>
> Eight VMs configured with varying numbers of VCPUs ranging from 1 to
> 8. Each VM executing a program with the same number of threads as it
> has VCPUs (1 VCPU VM has 1 thread, 8 VCPU VM has 8 threads) where each
> thread is running an infinite loop designed to use CPU time. No cap
> was set and each VM had a weight of 256.
>
> >From what I understand about how the credit scheduler works I would
> think in this case each VM would receive 12.5% of the total system CPU
> time. However, after running this test for a couple of hours the host
> CPU time had been allocated as follows:
>
> 1-VCPU VM: 12.14%
> 2-VCPU VM: 9.26%
> 3-VCPU VM: 11.58%
> 4-VCPU VM: 12.81%
> 5-VCPU VM: 13.35%
> 6-VCPU VM: 13.53%
> 7-VCPU VM: 13.62%
> 8-VCPU VM: 13.72%
>
> As you can see the number of VCPUs changes the allocation of CPU so
> that VMs with fewer VCPUs receive less CPU time than they should based
> on the configured weight value. I'm not sure why the 1-VCPU VM is
> getting more CPU time in this test than the 2 and 3 VCPU VMs. Overall
> the trend that I have seen is that assigning more VCPUs to a VM
> slightly increases that VM's priority on an overcommitted host, this
> test ended up with the 1-VCPU VM not following that trend exactly.
>
> I'd be interested to hear any thoughts you have on these results;
> either comments about my experiment setup, or thoughts about the why
> the scheduling algorithm is exhibiting this behavior.
>
> Thanks,
>
> -Gabriel
>
> On Mon, Jul 21, 2008 at 5:00 PM, Mark Williamson
> <mark.williamson@xxxxxxxxxxxx> wrote:
>> Hi Gabriel,
>>
>> I'm not particularly familiar with the credit scheduler but I'll do my best
>> to
>> help clarify things a bit (I hope!).
>>
>> On Thursday 03 July 2008, Gabriel Southern wrote:
>>> Hi,
>>>
>>> I'm working a project with SMP VMs and I noticed something about the
>>> behavior of the credit scheduler that does not match my understanding
>>> of the documentation about the credit scheduler. It seems like
>>> assigning more VCPUs to a VM increases the proportion of total system
>>> CPU resources the VM will receive, whereas the documentation indicates
>>> that this should be controlled by the weight value.
>>>
>>> For example when running a CPU intensvie benchmark with some VMs
>>> configured with 1-VCPU and other VMs configured with 8-VCPUs, the
>>> benchmark took 37% longer to complete on the VMs with 1-VCPU than the
>>> ones with 8-VCPUs. Unfortunately I did not record the exact values
>>> for CPU time that each VM received; however, I think that the 8-VCPU
>>> VMs did receive around 30% more CPU time than the 1-VCPU VMs. These
>>> tests were performed with the default weight of 256 for all VMs and no
>>> cap configured.
>>
>> You need to tell us a bit more about how you did your benchmarking... Were
>> the SMP and UP guests running concurrently and competing for CPU time? Or
>> were they run separately? Was the benchmark able to take advantage of
>> multiple CPUs itself?
>>
>>> I don't think that this is the behavior that the scheduler should
>>> exhibit based on the documentation I read. I admit the tests I was
>>> doing were not really practical use cases for real applications. But
>>> I'd be curious if anyone knows if this is a limitation of the design
>>> of the credit scheduler, or possibly due to a configuration problem
>>> with my system. I running Xen 3.2.0 compiled from the official source
>>> distribution tarball, and the guest VMs are also using the 3.2.0
>>> distribution with the 2.6.18 kernel. Any ideas anyone has about why
>>> my system is behaving this way are appreciated.
>>
>> Without knowing more about your setup there are lots of things that could be
>> happening...
>>
>> If you're not using caps then there's no reason why the SMP guests shouldn't
>> get more CPU time if they're somehow able to consume more slack time in the
>> system. SMP scheduling makes things pretty complicated!
>>
>> If you reply with more details, I can try and offer my best guess as to what
>> might be happening. If you don't get a response within a day or two, please
>> feel free to poke me directly.
>>
>> Cheers,
>> Mark
>>
>>>
>>> Thanks,
>>>
>>> Gabriel
>>>
>>> _______________________________________________
>>> Xen-devel mailing list
>>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>>> http://lists.xensource.com/xen-devel
>>
>>
>>
>> --
>> Push Me Pull You - Distributed SCM tool
>> (http://www.cl.cam.ac.uk/~maw48/pmpu/)
>>
>
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>
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