On Thu, 2011-03-24 at 08:47 +0100, Luc MAIGNAN wrote: > Yes, indeed.... > > I've modified the number of cpus via the command line : > > VBoxManage modifyvm MyVM --cpus x > > From 3 cpus, the performances go down... But I don't really understand > why... If someone can give me ways of explanations... Sure...! This has to do with the way hypervisor schedulers work with virtual SMB. In your case, the VM you are running may simply not need or use more than 2 CPU cores, or assigning 3 or more cores causes the scheduler to have to prioritize things in a less efficient way. Perhaps both are happening as well. Here's how this works: Basically, a multi-vCPU VM has to be scheduled for the same number of CPU cores for every CPU cycle (2 vCPUs, 2 cores; 3 vCPUs, 3 cores; and so forth). And this part is absolutely key: This gets done regardless of if there is actually anything for these cores to do. So, if you have a 4 vCPU VM that only has work to do for a single core, you still have to schedule 4 cores - of which 3 cores will be scheduled to basically do ...NOTHING! These cores cannot be used to perform other work during this time. I hope you can see that scheduling CPU cores to do nothing could cause a performance decrease. You basically are forcing the scheduler to choose which will do nothing as well. Further, scheduling vCPUs in odd numbers (like 3 vCPUs on a quad-core processor) causes other effects on performance due to scheduling conflicts. Say, for example, I have a 3 vCPU VM and a 2 vCPU VM running on a quad-core processor. I can never schedule these two to execute at the same time because I don't have enough cores. So, the general rule of thumb for how many vCPUs you need on a VM goes something like this: Highly threaded systems and applications will tend to need - and use - more cores, so configure more vCPUs for them. Things that tend to be heavy on calculations will tend to need fewer cores. Also, configure your VMs taking the total number of cores you have on the system into account so as to minimize possible scheduling conflicts. I usually use multiples of 2 vCPUs (1, 2, 4, 8) and avoid odd numbered vCPU configurations. That's especially true on quad-core processors. Six-core processors give you a little more flexibility (you could do combinations of 1, 2, 4, 6), but the same general rule still applies. In any case, make sure you understand the actual CPU needs of the system and application you are running. I have seen a single vCPU VM use more processor time on a dual-socket, quad-core box than an 8-way VM running on the same machine at the same time. In that case, the 8-way VM was happy as a clam because it wasn't that busy, but the single vCPU VM was redlining the processor because it wanted even more than what physical resources were available. Hope that helps explain things for you! Chris -- ============================== "If you are calm while all around you is chaos, then you probably haven't fully understood the magnitude of the situation." --Unknown -- users mailing list users@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe or change subscription options: https://admin.fedoraproject.org/mailman/listinfo/users Guidelines: http://fedoraproject.org/wiki/Mailing_list_guidelines
