Re: HVM guest performance regression

[Juergen Gross <jgross@suse.com>]

On 26/05/17 19:04, Dario Faggioli wrote:
> On Fri, 2017-05-26 at 18:14 +0200, Juergen Gross wrote:
>> Looking for the reason of a performance regression of HVM guests
>> under
>> Xen 4.7 against 4.5 I found the reason to be commit
>> c26f92b8fce3c9df17f7ef035b54d97cbe931c7a ("libxl: remove
>> freemem_slack")
>> in Xen 4.6.
>>
>> The problem occurred when dom0 had to be ballooned down when starting
>> the guest. The performance of some micro benchmarks dropped by about
>> a factor of 2 with above commit.
>>
> Performance of micro benchmarks run _inside_ the guest, I'm guessing?

Yep. libmicro benchmark "munmap".

>> Interesting point is that the performance of the guest will depend on
>> the amount of free memory being available at guest creation time.
>> When there was barely enough memory available for starting the guest
>> the performance will remain low even if memory is being freed later.
>>
> OOC, what kind of host? Big? Small? NUMA, non-NUMA?, etc

I've tested this to happen _always_ on my laptop (Dual core Intel(R)
Core(TM) i7-4600M CPU @ 2.90GHz, 8GB memory, non-NUMA).

Guest size was 2GB, 1 vcpu.


Juergen

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