|Subject:||[Qemu-discuss] QEMU MicroCheckpointing Pause & Resume Latency|
|Date:||Thu, 9 Mar 2017 16:45:58 +0800|
Dear QEMU users,
Currently I am working on a project related to QEMU MicroCheckpointing and I have encountered a performance issue during checkpoint pause & resume.
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
Thanks and best regards,
Niko Jiasheng Feng
University of Hong Kong
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