performance save/restore under xen-4.3.2 compared to kvm/qemu

performance save/restore under xen-4.3.2 compared to kvm/qemu

[ustermann.max]

Hello everybody,

i hope, i am right here with my question.

i have an vm with 1 GB main memory under Xen-4.3.2, if i measure the times for save and restore via "time", i got the following values:

save:
real    0m12.136s
user    0m0.175s
sys     0m2.662s
                                                
restore:
real    0m8.639s
user    0m0.468s
sys     0m1.807s


if i do the same with an vm under kvm/qemu (1GB main memory), i got this values:

save:
real    0m10.024s
user    0m0.008s
sys     0m0.003s

restore:
real    0m0.525s
user    0m0.015s
sys     0m0.004s


the host hardware is in both cases the same.
i´m real surprise about the huge difference for the needed time for restore, also that xen use much more time in kernel-mode (sys).
Can anyone give me some hints from where this difference can came?
Is there a way to speedup the restore-process in xen?

I´m thankful for every hint

all the best 
max

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Re: performance save/restore under xen-4.3.2 compared to kvm/qemu

[Andrew Cooper]

On 25/08/14 13:06, ustermann.max@web.de wrote:
> Hello everybody,
>
> i hope, i am right here with my question.
>
> i have an vm with 1 GB main memory under Xen-4.3.2, if i measure the times for save and restore via "time", i got the following values:
>
> save:
> real    0m12.136s
> user    0m0.175s
> sys     0m2.662s
>                                                 
> restore:
> real    0m8.639s
> user    0m0.468s
> sys     0m1.807s
>
>
> if i do the same with an vm under kvm/qemu (1GB main memory), i got this values:
>
> save:
> real    0m10.024s
> user    0m0.008s
> sys     0m0.003s
>
> restore:
> real    0m0.525s
> user    0m0.015s
> sys     0m0.004s
>
>
> the host hardware is in both cases the same.
> i´m real surprise about the huge difference for the needed time for restore, also that xen use much more time in kernel-mode (sys).
> Can anyone give me some hints from where this difference can came?
> Is there a way to speedup the restore-process in xen?
>
> I´m thankful for every hint
>
> all the best 
> max

Xen and KVM are two different types of hypervisor.  By its nature,
kvm/qemu has less to do for migration, as it already has full access to
the VMs memory.


It looks plausibly like qemu restore is mmap()ing the restore file and
running straight from there.  You are never going to manage this under
Xen, because of the extra isolation inherent in the Xen model.

In terms of raw speed, my migration v2 series (still in development) has
fixed several performance problems in the old migration code 
Particularly in the case of your example, my new code will be 4 times
faster as it is not mapping everything up to 4GB in the VM.

I have also identified a bottleneck in the Linux PVOps kernel where the
mmap batch ioctl takes a batch size of 1024 and generates 1024 batch
hypercalls of batch size 1.  Fixing this will certainly make the
mapping/unmapping faster.

~Andrew

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Re: performance save/restore under xen-4.3.2 compared to kvm/qemu

[Jan Beulich]

>>> On 25.08.14 at 14:23, <andrew.cooper3@citrix.com> wrote:
> I have also identified a bottleneck in the Linux PVOps kernel where the
> mmap batch ioctl takes a batch size of 1024 and generates 1024 batch
> hypercalls of batch size 1.  Fixing this will certainly make the
> mapping/unmapping faster.

Didn't Mats Petersson post a patch for this quite a while ago? Did
this perhaps never get applied?

Jan

Re: performance save/restore under xen-4.3.2 compared to kvm/qemu

[Andrew Cooper]

On 25/08/14 13:35, Jan Beulich wrote:
>>>> On 25.08.14 at 14:23, <andrew.cooper3@citrix.com> wrote:
>> I have also identified a bottleneck in the Linux PVOps kernel where the
>> mmap batch ioctl takes a batch size of 1024 and generates 1024 batch
>> hypercalls of batch size 1.  Fixing this will certainly make the
>> mapping/unmapping faster.
> Didn't Mats Petersson post a patch for this quite a while ago? Did
> this perhaps never get applied?
>
> Jan
>

That was a different issue if I remember correctly.  I think it was to
do with the classic kernel using ioremap() for foreign domain memory.

~Andrew

Re: performance save/restore under xen-4.3.2 compared to kvm/qemu

[Jan Beulich]

>>> On 25.08.14 at 14:38, <andrew.cooper3@citrix.com> wrote:
> On 25/08/14 13:35, Jan Beulich wrote:
>>>>> On 25.08.14 at 14:23, <andrew.cooper3@citrix.com> wrote:
>>> I have also identified a bottleneck in the Linux PVOps kernel where the
>>> mmap batch ioctl takes a batch size of 1024 and generates 1024 batch
>>> hypercalls of batch size 1.  Fixing this will certainly make the
>>> mapping/unmapping faster.
>> Didn't Mats Petersson post a patch for this quite a while ago? Did
>> this perhaps never get applied?
> 
> That was a different issue if I remember correctly.  I think it was to
> do with the classic kernel using ioremap() for foreign domain memory.

I certainly remember that patch having been against pv-ops...

Jan

Re: performance save/restore under xen-4.3.2 compared to kvm/qemu

[max ustermann]

 Hi,

first, thank you for the Information.
For clearness i will comment, that i did the tests with an HVM Guest (Windows XP) not with an PV-Guest. Make this any diffenrence to your explanations?

and then i would like to ask, are your modifications anywere avaible? are they in the unstable-tree of xen ????

all the best
max
 
----------------ursprüngliche Nachricht-----------------
Von: "Andrew Cooper" andrew.cooper3@citrix.com 
An: ustermann.max@web.de, xen-devel@lists.xen.org 
Datum: Mon, 25 Aug 2014 13:23:09 +0100
-------------------------------------------------
 
 
> On 25/08/14 13:06, ustermann.max@web.de wrote:
>> Hello everybody,
>>
>> i hope, i am right here with my question.
>>
>> i have an vm with 1 GB main memory under Xen-4.3.2, if i measure the times for 
>> save and restore via "time", i got the following values:
>>
>> save:
>> real 0m12.136s
>> user 0m0.175s
>> sys 0m2.662s
>> 
>> restore:
>> real 0m8.639s
>> user 0m0.468s
>> sys 0m1.807s
>>
>>
>> if i do the same with an vm under kvm/qemu (1GB main memory), i got this values:
>>
>> save:
>> real 0m10.024s
>> user 0m0.008s
>> sys 0m0.003s
>>
>> restore:
>> real 0m0.525s
>> user 0m0.015s
>> sys 0m0.004s
>>
>>
>> the host hardware is in both cases the same.
>> i´m real surprise about the huge difference for the needed time for restore, 
>> also that xen use much more time in kernel-mode (sys).
>> Can anyone give me some hints from where this difference can came?
>> Is there a way to speedup the restore-process in xen?
>>
>> I´m thankful for every hint
>>
>> all the best 
>> max
> 
> Xen and KVM are two different types of hypervisor. By its nature,
> kvm/qemu has less to do for migration, as it already has full access to
> the VMs memory.
> 
> 
> It looks plausibly like qemu restore is mmap()ing the restore file and
> running straight from there. You are never going to manage this under
> Xen, because of the extra isolation inherent in the Xen model.
> 
> In terms of raw speed, my migration v2 series (still in development) has
> fixed several performance problems in the old migration code 
> Particularly in the case of your example, my new code will be 4 times
> faster as it is not mapping everything up to 4GB in the VM.
> 
> I have also identified a bottleneck in the Linux PVOps kernel where the
> mmap batch ioctl takes a batch size of 1024 and generates 1024 batch
> hypercalls of batch size 1. Fixing this will certainly make the
> mapping/unmapping faster.
> 
> ~Andrew
> 

-- 




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Re: performance save/restore under xen-4.3.2 compared to kvm/qemu

[Andrew Cooper]

On 25/08/14 14:35, max ustermann wrote:
>  Hi,
>
> first, thank you for the Information.
> For clearness i will comment, that i did the tests with an HVM Guest (Windows XP) not with an PV-Guest. Make this any diffenrence to your explanations?

Not at all.  My explanation still stands, especially the bit about 4
times faster.

>
> and then i would like to ask, are your modifications anywere avaible? are they in the unstable-tree of xen ????

I am working to try and get them included in Xen-4.5 before the code
freeze, but there is currently nothing in xen-unstable. 
Work-in-progress is available on my personal xenbits tree:

http://xenbits.xen.org/gitweb/?p=people/andrewcoop/xen.git;a=shortlog;h=refs/heads/saverestore2-v6.1

~Andrew

Re: performance save/restore under xen-4.3.2 compared to kvm/qemu

[David Vrabel]

On 25/08/14 13:35, Jan Beulich wrote:
>>>> On 25.08.14 at 14:23, <andrew.cooper3@citrix.com> wrote:
>> I have also identified a bottleneck in the Linux PVOps kernel where the
>> mmap batch ioctl takes a batch size of 1024 and generates 1024 batch
>> hypercalls of batch size 1.  Fixing this will certainly make the
>> mapping/unmapping faster.
> 
> Didn't Mats Petersson post a patch for this quite a while ago? Did
> this perhaps never get applied?

I think it needed some reworking and no one else has picked it up yet.

David