Re: [for-4.9] Re: HVM guest performance regression

[Juergen Gross <jgross@suse.com>]

On 07/06/17 20:19, Stefano Stabellini wrote:
> On Wed, 7 Jun 2017, Juergen Gross wrote:
>> On 06/06/17 21:08, Stefano Stabellini wrote:
>>> On Tue, 6 Jun 2017, Juergen Gross wrote:
>>>> On 06/06/17 18:39, Stefano Stabellini wrote:
>>>>> On Tue, 6 Jun 2017, Juergen Gross wrote:
>>>>>> On 26/05/17 21:01, Stefano Stabellini wrote:
>>>>>>> On Fri, 26 May 2017, Juergen Gross wrote:
>>>>>>>> On 26/05/17 18:19, Ian Jackson wrote:
>>>>>>>>> Juergen Gross writes ("HVM guest performance regression"):
>>>>>>>>>> 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.
>>>>>>>>>>
>>>>>>>>>> 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.
>>>>>>>>>>
>>>>>>>>>> I'd like to suggest we either revert the commit or have some other
>>>>>>>>>> mechanism to try to have some reserve free memory when starting a
>>>>>>>>>> domain.
>>>>>>>>>
>>>>>>>>> Oh, dear.  The memory accounting swamp again.  Clearly we are not
>>>>>>>>> going to drain that swamp now, but I don't like regressions.
>>>>>>>>>
>>>>>>>>> I am not opposed to reverting that commit.  I was a bit iffy about it
>>>>>>>>> at the time; and according to the removal commit message, it was
>>>>>>>>> basically removed because it was a piece of cargo cult for which we
>>>>>>>>> had no justification in any of our records.
>>>>>>>>>
>>>>>>>>> Indeed I think fixing this is a candidate for 4.9.
>>>>>>>>>
>>>>>>>>> Do you know the mechanism by which the freemem slack helps ?  I think
>>>>>>>>> that would be a prerequisite for reverting this.  That way we can have
>>>>>>>>> an understanding of why we are doing things, rather than just
>>>>>>>>> flailing at random...
>>>>>>>>
>>>>>>>> I wish I would understand it.
>>>>>>>>
>>>>>>>> One candidate would be 2M/1G pages being possible with enough free
>>>>>>>> memory, but I haven't proofed this yet. I can have a try by disabling
>>>>>>>> big pages in the hypervisor.
>>>>>>>
>>>>>>> Right, if I had to bet, I would put my money on superpages shattering
>>>>>>> being the cause of the problem.
>>>>>>
>>>>>> Seems you would have lost your money...
>>>>>>
>>>>>> Meanwhile I've found a way to get the "good" performance in the micro
>>>>>> benchmark. Unfortunately this requires to switch off the pv interfaces
>>>>>> in the HVM guest via "xen_nopv" kernel boot parameter.
>>>>>>
>>>>>> I have verified that pv spinlocks are not to blame (via "xen_nopvspin"
>>>>>> kernel boot parameter). Switching to clocksource TSC in the running
>>>>>> system doesn't help either.
>>>>>
>>>>> What about xen_hvm_exit_mmap (an optimization for shadow pagetables) and
>>>>> xen_hvm_smp_init (PV IPI)?
>>>>
>>>> xen_hvm_exit_mmap isn't active (kernel message telling me so was
>>>> issued).
>>>>
>>>>>> Unfortunately the kernel seems no longer to be functional when I try to
>>>>>> tweak it not to use the PVHVM enhancements.
>>>>>
>>>>> I guess you are not talking about regular PV drivers like netfront and
>>>>> blkfront, right?
>>>>
>>>> The plan was to be able to use PV drivers without having to use PV
>>>> callbacks and PV timers. This isn't possible right now.
>>>
>>> I think the code to handle that scenario was gradually removed over time
>>> to simplify the code base.
>>
>> Hmm, too bad.
>>
>>>>>> I'm wondering now whether
>>>>>> there have ever been any benchmarks to proof PVHVM really being faster
>>>>>> than non-PVHVM? My findings seem to suggest there might be a huge
>>>>>> performance gap with PVHVM. OTOH this might depend on hardware and other
>>>>>> factors.
>>>>>>
>>>>>> Stefano, didn't you do the PVHVM stuff back in 2010? Do you have any
>>>>>> data from then regarding performance figures?
>>>>>
>>>>> Yes, I still have these slides:
>>>>>
>>>>> https://www.slideshare.net/xen_com_mgr/linux-pv-on-hvm
>>>>
>>>> Thanks. So you measured the overall package, not the single items like
>>>> callbacks, timers, time source? I'm asking because I start to believe
>>>> there are some of those slower than their non-PV variants.
>>>
>>> There isn't much left in terms of individual optimizations: you already
>>> tried switching clocksource and removing pv spinlocks. xen_hvm_exit_mmap
>>> is not used. Only the following are left (you might want to double check
>>> I haven't missed anything):
>>>
>>> 1) PV IPI
>>
>> Its a 1 vcpu guest.
>>
>>> 2) PV suspend/resume
>>> 3) vector callback
>>> 4) interrupt remapping
>>>
>>> 2) is not on the hot path.
>>> I did individual measurements of 3) at some points and it was a clear win.
>>
>> That might depend on the hardware. Could it be newer processors are
>> faster here?
> 
> I don't think so: the alternative it's an emulated interrupt. It's
> slower under all points of view.

What about APIC virtualization of modern processors? Are you sure e.g.
timer interrupts aren't handled completely by the processor? I guess
this might be faster than letting it be handled by the hypervisor and
then use the callback into the guest.

> I would try to run the test with xen_emul_unplug="never" which means
> that you are going to end up using the emulated network card and
> emulated IDE controller, but some of the other optimizations (like the
> vector callback) will still be active.

Now this is something I wouldn't like to do. My test isn't using any
I/O at all and is showing bad performance with pv interfaces being used.
The only remedy right now seems to be to switch off pv interfaces
leading to a bad I/O performance, but a good non-I/O performance.

You are suggesting a mode with bad I/O performance _and_ bad non-I/O
performance.

> If the cause of the problem is ballooning for example, using emulated
> interfaces for IO will reduce the amount of ballooned out pages
> significantly.

No I/O involved in my benchmark.


Juergen

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