VirtualBox 4.2.4 on FreeBSD 9.1-PRERELEASE problem: VMs behave very different when pinned to different cores

[Alex Chistyakov]

On Fri, Nov 23, 2012 at 6:20 PM, Bernhard Fröhlich <decke at freebsd.org> wrote:
> On Fri, Nov 23, 2012 at 2:15 PM, Alex Chistyakov <alexclear at gmail.com> wrote:
>> Hello,
>>
>> I am back with another problem. As I discovered previously setting a
>> CPU affinity explicitly helps to get decent performance on guests, but
>> the problem is that guest performance is very different on core #0 and
>> cores #5 or #7. Basically when I use 'cpuset -l 0 VBoxHeadless -s
>> "Name" -v on' to start the VM it is barely usable at all. The best
>> performance results are on cores #4 and #5 (I believe they are the
>> same physical core due to HT). #7 and #8 are twice as slow as #5, #0
>> and #1 are the slowest and other cores lay in the middle.
>> If I disable a tickless kernel on the guest running on #4 or #5 it
>> becomes as slow as a guest running on #7 so I suspect this is a
>> timer-related issue.
>> I also discovered that there are quite a lot of system interrupts on
>> slow guests (%si is about 10-15) but Munin does not render them on its
>> CPU graphs for some reason.
>> All my VMs are on cores #4 and #5 right now but I want to utilize
>> other cores too. I am not sure what to do next, this looks like a
>> VirtualBox bug. What can be done to solve this?
>
> I do not want to sound ignorant but what do you expect? Each VBox
> VM consists of somewhere around 15 threads and some of them are the
> vCPUs. You bind them all to the same CPU so they will fight for CPU time
> on that single core and latency will get unpredictable as well as
> performance. And then you add more and more craziness by running
> it on cpu0 and a HT enabled CPU ...

Your point regarding HTT is perfectly valid so I just disabled it in
BIOS. Unfortunately it did not help.
When I run a single VM on CPU #0 I get the following load pattern on the host:

last pid:  2744;  load averages:  0.93,  0.63,  0.31

              up 0+00:05:25  19:37:17
368 processes: 8 running, 344 sleeping, 16 waiting
CPU 0: 14.7% user,  0.0% nice, 85.3% system,  0.0% interrupt,  0.0% idle
CPU 1:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
CPU 2:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
CPU 3:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
CPU 4:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
CPU 5:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
Mem: 410M Active, 21M Inact, 921M Wired, 72K Cache, 60G Free
ARC: 136M Total, 58M MRU, 67M MFU, 272K Anon, 2029K Header, 8958K Other
Swap: 20G Total, 20G Free

And when I run it on CPU #4 the situation is completely different:

last pid:  2787;  load averages:  0.05,  0.37,  0.31

              up 0+00:11:45  19:43:37
368 processes: 9 running, 343 sleeping, 16 waiting
CPU 0:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
CPU 1:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
CPU 2:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
CPU 3:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
CPU 4:  1.8% user,  0.0% nice, 11.0% system,  0.0% interrupt, 87.2% idle
CPU 5:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
Mem: 412M Active, 20M Inact, 1337M Wired, 72K Cache, 60G Free
ARC: 319M Total, 136M MRU, 171M MFU, 272K Anon, 2524K Header, 9340K Other
Swap: 20G Total, 20G Free

Regarding pinning the VM to a certain core - yes, I agree with you,
it's better not to pin VMs explicitly but I was forced to do this. If
I do not pin the VM explicitly it gets scheduled to the "bad" core
sooner or later and the whole VM gets unresponsive. And I was able to
run as many as 6 VMs on HTT cores #4/#5 quite successfully. These VMs
were staging machines without too much load on them but I wanted to
put some production resources on this host too - that's why I wanted
to know how to utilize other cores safely.

Thank you,

--
SY,
Alex