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 <dgilb...@redhat.com > wrote: > * FENG, Jiasheng (nikof...@connect.hku.hk) 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: nikof...@hku.hk / niko_jiash...@163.com > -- > Dr. David Alan Gilbert / dgilb...@redhat.com / 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: nikof...@hku.hk / niko_jiash...@163.com
