(cc'ing in Paolo since he knows our barrier code) * FENG, Jiasheng (nikof...@connect.hku.hk) wrote: > 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
Just checking, is this on a normal x86 PC? Your numbers of 3-5ms just seem quite high to me but I've not tried timing that code. Dave > > 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 -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
