On 06/19/2009 04:15 PM, Daniel P. Berrange wrote:
On Fri, Jun 19, 2009 at 03:59:57PM +0200, Jes Sorensen wrote:
Hi,
This one introduces a -maxcpus setting, allowing the user to specify
the maximum number of vCPUs the system can have, as discussed with Gleb
earlier in the week.
What is the maximum value for the 'maxcpus' setting for KVM ?
Right now it is still 16, however I plan to change this.
libvirt currently does
fd = open("/dev/kvm")
r = ioctl(fd, KVM_CHECK_EXTENSION, KVM_CAP_NR_VCPUS);
to figure out what the maximum allowed vCPUs will be for KVM,
and currently it is returning 16 IIRC.
Interesting, this will need to be addressed as well. I have plans to
introduce a mechanism telling the kernel where the limit will be, in
order to allow it to allocate data structures in a reasonable manner.
@@ -5666,6 +5667,13 @@ int main(int argc, char **argv, char **e
exit(1);
}
break;
+ case QEMU_OPTION_maxcpus:
+ max_cpus = atoi(optarg);
+ if ((max_cpus< 1) || (max_cpus> machine->max_cpus)) {
+ fprintf(stderr, "Invalid number of CPUs\n");
+ exit(1);
+ }
This implies the limit (for x86 pc machine at least) is now 255. Is that
the correct interpretation on my part ?
Actually the 255 limit is tied to ACPI. Once we support ACPI 3.0 and
x2apic, it will get much worse. Be afraid, be very afraid :-) To be
honest, that is going to be a while before we get to that, but I hope
to get to it eventually.
I strongly recommend you try not to impose static limits within libvirt
for the number of vCPUs.
I guess it will become a tricky issue who is telling who what the limit
is. Ideally I would like to see the kernel limit becoming unlimited and
the restrictions being set by userland.
Cheers,
Jes
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