[kernel-xen] Maximum number of CPUs set into 16 (CONFIG_NR_CPUS=16) ?

[Yuriy Kohut]

Hi, Steven

I faced with the issue yesterday. HV has 2 following processors:

processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 45
model name      : Intel(R) Xeon(R) CPU E5-2620 0 @ 2.00GHz
stepping        : 7
microcode       : 0x70b
cpu MHz         : 2000.052
cache size      : 15360 KB
physical id     : 0
siblings        : 12
core id         : 0
cpu cores       : 6
apicid          : 0
initial apicid  : 0
fpu             : yes
fpu_exception   : yes
cpuid level     : 13
wp              : yes
flags           : fpu de tsc msr pae mce cx8 apic sep mca cmov pat clflush acpi mmx fxsr sse sse2 ss ht syscall nx lm constant_tsc rep_good nopl nonstop_tsc eagerfpu pni pclmulqdq est ssse3 cx16 sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx hypervisor lahf_lm ida arat epb xsaveopt pln pts dtherm
bogomips        : 4000.10
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

This is 6 core (12 threads) CPUs.

So we should get 12 cores without threading and 24 with threading.

Here is what we get:
[root at CC-HVXS-2 ~]# grep processor /proc/cpuinfo 
processor    : 0
processor    : 1
processor    : 2
processor    : 3
processor    : 4
processor    : 5
processor    : 6
processor    : 7
processor    : 8
processor    : 9
processor    : 10
processor    : 11
processor    : 12
processor    : 13
processor    : 14
processor    : 15

This result is greater then 12 - so it is with threading. As the result Dom0 kernel will use only 16 threaded kernels out of 24.
The other thing is, I'm NOT 100% sure that all vCPUs (not only 16) will be available for DomUs.

And the last thing is, that I don't understand why 16 ? 
While kernel.org sets as default 64, and RedHat 5.x default is 255...

So, is there any chance to get the number updated in upcoming kernel-xen RPM(s) ?

---
Yura

On Jan 24, 2013, at 5:19 AM, Steven Haigh <netwiz at crc.id.au> wrote:

> On 24/01/2013 2:16 AM, Yuriy Kohut wrote:
>> Hi,
>> 
>> What is the reason to set the maximum number of CPUs into 16 ?
>> 
>> The latest kernel-xen-3.7.4-1.el6xen.src.rpm package includes the config
>> with:
>> # grep -r CONFIG_NR_CPUS rpmbuild
>> rpmbuild/SOURCES/config-3.7.x86_64:CONFIG_NR_CPUS=16
>> 
>> While there is 64 CPU allowed in the default x86_64 config on kernel.org
>> <http://kernel.org>):
>> 
>> http://git.kernel.org/?p=linux/kernel/git/stable/linux-stable.git;a=blob_plain;f=arch/x86/configs/x86_64_defconfig;hb=8a69ca20f06d1878aa0ee8000a60d8226ab6b6a1
> 
> I don't think anyone has hit the 16 CPU limit - nor would anyone be likely to hit it...
> 
> Keep in mind that the number here is ONLY for the number of VCPUs that Dom0 has. The hardware may have 64 CPU cores - but only 1-2 allocated for Dom0. I really doubt any sane configuration would have more than 16 VCPUs set for Dom0.
> 
> -- 
> Steven Haigh
> 
> Email: netwiz at crc.id.au
> Web: http://www.crc.id.au
> Phone: (03) 9001 6090 - 0412 935 897
> Fax: (03) 8338 0299
> 
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