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Re: [Qemu-devel] QEMU MicroCheckpointing Pause & Resume Latency
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] QEMU MicroCheckpointing Pause & Resume Latency |
Date: |
Thu, 9 Mar 2017 15:19:23 +0000 |
User-agent: |
Mutt/1.7.1 (2016-10-04) |
* 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