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xen-devel
Re: [Xen-devel] Priority for SMP VMs
Those are certainly unexpected results. :-) Hmm... I'll take a quick
look sometime this week or next. I have some internal patches that add
tracing to runstate changes, and an internal tool that's not really
ready for release yet that can do all sorts of fun analysis... let me
know if you want the patches. (I'll probably try to get the patches in
after the freeze is up.)
-George
Gabriel Southern wrote:
Hi George,
Thanks for your comments. I understand that the scheduler has to
balance a variety of different VM activity and I am only testing one
very limited aspect of it. I tried running the test you suggested
using just "while(1) ;" loops and making sure I had enough threads
running so that each VM could use all the CPU time it had available.
The CPU time allocation was basically the same as what I described
earlier:
1-VCPU VM 12.28%
2-VCPU VM 9.26%
3-VCPU VM 11.55%
4-VCPU VM 12.79%
5-VCPU VM 13.32%
6-VCPU VM 13.50%
7-VCPU VM 13.60%
8-VCPU VM 13.71%
I also tried running a test with 8 VM where 7-VMs had 8-VCPUs and 1 VM
had 1-VCPU. Each VM was running 8 threads of the "while (1) ;" loops
to make sure it was trying to use all the CPU time it could get. In
this case each of the 8-VCPU VMs received around 12.96% of CPU time
and the 1-VCPU VM received 9.27%.
I have a basic idea about how the credit scheduler works, but not good
enough to understand exactly why I am seeing this behavior. I'm
guessing it has to do with the VMs that have more VCPUs getting extra
opportunities to run simply because they have more entries in the
runq.
I'd be curious if anyone else is able to verify the behavior I've
described. Also if anyone who has a better understanding of how the
credit scheduler has a better idea of why I'm observing this behavior
I'd be interested to hear that as well. Obviously I don't think this
is a high priority problem, but it might be something that is useful
to be aware of. I also admit that I could be observing this behavior
due to some sort of user error on my part, rather than there being any
problem with the credit scheduler.
Thanks,
Gabriel
On Tue, Jul 22, 2008 at 7:07 AM, George Dunlap
<George.Dunlap@xxxxxxxxxxxxx> wrote:
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
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