[vbox-dev] questions about number of CPUs and host scheduling behaviour

Nikolay Igotti nikolay.igotti at oracle.com
Wed May 11 11:29:20 GMT 2011

Bayard Bell wrote:
>> CPU isn't that easily "given" to the VM, as RAM pages, for example. VirtualBox internally need to run few threads doing disk/network IO. Same situation with host OS too, so essentially some experiments is the best way to figure out how many vCPUs is reasonable to give to the guest to get best performance.
> Any suggestions as to how to go about that methodically? 
 Well, just try some representative subset (10-20 mins of compilation) 
with 1,2,3,4... vCPUs and see the result :).
Could be easily automated with vboxshell and guest commands execution 

> What I know is that the run queue seems to back up to the point of crushing the host if I provide only two vCPUs, while with 4 vCPUs, I only seem to get consumption of 2 actual CPUs. I've got a slight further wrinkle, insofar as the default behaviour of the build environment is to look at the number of CPUs and amount of memory and decide for itself what the appropriate level of parallelism is, although I can work around this by setting a fixed value before experimenting with CPU count. Just to give this a bottom line, if I haven't mentioned this previously: I've got a compile job that normally takes at most few hours on comparable bare metal, and it's taking several days under VBox. Resolving this is the difference between being able to get acceptably slower performance under VBox and needing to sort myself out with a separate system.
 Is project you're compiling open source? This could make analysis simpler.

>> With compilation, especially if you compile a lot of small files, significant part of load is fork/exec performance (and so, VMM in the guest), and of course, IO does matter too.
> The I/O is trivial, but what I'm gathering is that the CPU overhead of the system calls is increased considerably. I don't see a lot of fork and exec load, but what I'm wondering is whether time spent in the kernel would actually be relatively longer, such that relatively lightweight system calls on a normal host would add up to a considerably higher percentage of CPU time in a virtual environment.
 Syscalls per se aren't affected much by virtualization, but privileged 
operations they perform sometimes are.
Generally, this need deeper analysis, and you may want to try running 
same guest on different host OS (ideally with
the same hardware), to see if some host specifics presented.

Also no sure if OSX is best OS to run SMP load in general.

>> Don't think you really need that. As VBox doesn't do explicit gang scheduling, some assistance from host scheduler on that would be helpful, not explicit assignment of CPU affinity. In theory, good scheduler shall gang schedule threads with the same address space even without additional hints, as this will likely increase performance. Not sure if OSX does that, although.
> Thanks for that info. I'll see if there's any documentation or source to satisfy my curiosity on this point. It might also be useful to see what DTrace can tell me. Does VBox have its own DTrace probes to help with these kinds of problems?
   Don't think VBox has much of probes on its own, but even OS traces 
could be sufficiently useful.



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