Re: [Qemu-devel] QEMU MicroCheckpointing Pause & Resume Latency

[FENG, Jiasheng]

Dear David,

Really appreciate your feedback.

I have proceeded the experiments in both conditions, and no matter the
vCPUs are in idle or busy situation, there is no difference that smp_wmb()
will consume a lot of time to proceed its work.

In your opinion, may I know that what is the alternative way to minimize
the time consumption of smp_wmb() or any other system setting could speed
up smp_wmb()?

Thanks in advance for your assistance and hope to receive your feedback soon

Thanks and best regards,
Niko Jiasheng Feng



On Thu, Mar 9, 2017 at 11:19 PM, Dr. David Alan Gilbert <address@hidden
> wrote:

> * FENG, Jiasheng (address@hidden) wrote:
> > Dear QEMU Development Team,
> >
> >
> > It is my honor to contact with you.
> >
> >
> >
> > I am a postgraduate student from University of Hong Kong. Currently I am
> > working on a project related to QEMU MicroCheckpointing and I have
> > encountered a performance issue during checkpoint pause & resume.
>
> The microcheckpointing code hasn't been maintained for a long time;
> most of the current checkpointing work is based on the COLO work which is
> still under development.
>
> > Please kindly refer to migration/checkpoint.c file, in function
> > capture_checkpoint, I proceeded a test to see the time consumption
> between
> > vm_stop_force_state and vm_start. I found out that even if the system is
> > idle, there are still 12-20ms latency recorded ( mem=2G, vCPU=4 ).
> > Moreover, latency will be increased while more cpus equipped by my
> virtual
> > machine. I have done some research on that and I realized that it is
> > related to the Memory Barrier in KVM kernel. Each cpu will proceed a
> > smp_wmb() request during pause & resume and it takes about  3-5ms to
> finish
> > the request ( mem=2G, vCPU=4 ).
> >
> >
> >
> > Therefore, I would like to ask 3 questions regarding on the above issue:
> >
> >
> > 1. What is your consideration with calling smp_wmb() in checkpoint
> period;
> >
> > 2. Is it any other solution to minimize the latency to improve the
> > performance in checkpoint period;
> >
> > 3. Is smp_wmb() able to be safely disabled during the checkpoint period
>
> Well you'd have to understand where it's used; but for example, when taking
> a checkpoint you'd want to be sure that the checkpoint data contained
> a consistent copy of the last write data from all of the vCPUs; so I think
> a wmb would be needed to make sure it's consistent.
>
> I'm surprised that the smp_wmb is such a big chunk of your total checkpoint
> time, and that it's quite so long.
> Are the vCPUs idle or are they busy - does it make difference?
>
> Dave
>
> > Really appreciate your help with my problems and hope to receive your
> > feedback soon.
> >
> >
> > Thanks again for your contribution to QEMU and it is such a masterpiece.
>
> Dave
>
> >
> >
> >
> > Thanks and best regards,
> >
> > Niko Jiasheng Feng
> >
> > University of Hong Kong
> >
> > --
> > *Niko Jiasheng *
> > *Feng **Computer Science(General Stream), Faculty of Engineering, The
> > University of Hong Kong*
> > Contact:  （852）97908620
> > Address: Pokfulam Road, The University of Hong Kong
> > Email:      address@hidden / address@hidden
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK
>



-- 
*Niko Jiasheng *
*Feng **Computer Science(General Stream), Faculty of Engineering, The
University of Hong Kong*
Contact:  （852）97908620
Address: Pokfulam Road, The University of Hong Kong
Email:      address@hidden / address@hidden