Re: performance tuning, problem with paravirtualized clock

[Anthony Liguori <anthony@codemonkey.ws>]

Nikola Ciprich wrote:
> Hi Anthony!
> Anthony Liguori wrote:
>   
>> I would think you should get about 70% of native with what you've done 
>> about.  I've not seen instabilities with CONFIG_KVM_CLOCK myself.
>>
>> Setting up a hugetlbfs mount and using -mem-path may give you a bit of 
>> a bump too but I'd be surprised if it was more than 5%
>>     
> I've tried it now, and starting kvm with -mem-path pointing to hugetlbfs 
> mounted dir immediately fails and I see following message in dmesg of host:
> VM: killing process qemu-system-x86
> pointing to tmpfs mounted dir seems to work, I'll measure performance 
> gain...
>   

You won't see a gain with tmpfs.  Make sure you reserve huge pages 
first.  For a 1GB guest, you'll need something like:

echo 540 > /proc/sys/vm/nr_hugepages

When you create a VM, you need a bit more memory than 1GB for per-guest 
overhead.  That's why I reserve 540 instead of 512.  You can probably 
get away with 530 really.

Check that it succeeded by cat'ing /proc/meminfo.

>> The next biggest win you're going to see is using NPT (available in 
>> the recent AMD Barcelona/Phenom processors).   NPT + hugetblfs should 
>> get you pretty close to native (I'd reckon 95-98%).
>>     
> Yup, it seemed to me  that kvm performes WAY better on my phenom based 
> home desktop! I'll check that later too
>   

Definitely.  A parallel compile is one of the best case scenarios for 
NPT so you should see the most dramatic improvement there.

>> On the Intel side of things, you'll have to wait until the Nehalem 
>> which will support EPT (which is Intel's version of NPT).
>>
>> Can you be specific about your guest configurations?  Are you using 
>> -smp 8?
>>     
> yes, I'm using -smp 8
>   

It's not quite apples to apples then since you're sharing CPUs with the 
host.  Typically, if I'm benchmarking an 8-way system with 2GB of RAM, 
I'll create a 4-way guest with 1GB of RAM and then to generate native 
numbers, reboot the host with maxcpus=4 mem=1G.

Regards,

Anthony Liguori

>> Regards,
>>
>> Anthony Liguori
>>     
>
>
>
>
>
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