* 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
