Re: [Qemu-devel] Enabling Hyperthreading for Guest

[Erik Rull <erik.rull@rdsoftware.de>]

Alex Williamson wrote:
> On Thu, 2011-09-22 at 18:43 +0200, Erik Rull wrote:
>> Alex Williamson wrote:
>>> See the extended -smp options:
>>>
>>> -smp n[,maxcpus=cpus][,cores=cores][,threads=threads][,sockets=sockets]
>>>                   set the number of CPUs to 'n' [default=1]
>>>                   maxcpus= maximum number of total cpus, including
>>>                   offline CPUs for hotplug, etc
>>>                   cores= number of CPU cores on one socket
>>>                   threads= number of threads on one CPU core
>>>                   sockets= number of discrete sockets in the system
>>>
>>> Try something like:
>>>
>>> -smp 4,cores=2,threads=2,sockets=1
>>>
>>> Alex
>>>
>>
>> Great - the correct combination made it :-)
>>
>> But the SMP-Performance-Benchmark is horrible :-(
>> "Only" between 0.35 and 1.05 for the above combination.
>
> I'm not sure what that means...
>
>> I have the same architecture on the host (2 cores w/ ht enabled) so there
>> are enough real cores available for computation.
>>
>> Any idea what could slow down the performance here?
>
> Note that threads aren't real "full" cores, so you're likely going to
> see some scheduling mismatches between physical threads and virtual
> threads.  One thing that often helps smp guests is to pin vCPUs to
> pCPUs.  You can get the vCPU thread IDs from 'info cpu' in the monitor
> and pin each to a physical CPU with taskset.  If you're using libvirt,
> virt-manager can configure it to do this too (as well as the cpu
> topology).
>
> Alex
>

The SMP factor means how much speed improvement was gained using SMP 
against a single core with the same algorithm. My results showed a heavy 
performance breakdown when doing the SMP benchmark.
Fixing the virtual cores to the physical ones worked with taskset but 
didn't bring that real performance improvements at all :-(

Maybe the used algorithm for benchmarking is not the best one, I will try 
others.

Best regards,

Erik