* Daniel P. Berrangé (berrange@xxxxxxxxxx) wrote: > On Thu, Jun 28, 2018 at 05:12:39PM +0800, Xiao Guangrong wrote: > > > > Hi Peter, > > > > Sorry for the delay as i was busy on other things. > > > > On 06/19/2018 03:30 PM, Peter Xu wrote: > > > On Mon, Jun 04, 2018 at 05:55:14PM +0800, guangrong.xiao@xxxxxxxxx wrote: > > > > From: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxx> > > > > > > > > Detecting zero page is not a light work, we can disable it > > > > for compression that can handle all zero data very well > > > > > > Is there any number shows how the compression algo performs better > > > than the zero-detect algo? Asked since AFAIU buffer_is_zero() might > > > be fast, depending on how init_accel() is done in util/bufferiszero.c. > > > > This is the comparison between zero-detection and compression (the target > > buffer is all zero bit): > > > > Zero 810 ns Compression: 26905 ns. > > Zero 417 ns Compression: 8022 ns. > > Zero 408 ns Compression: 7189 ns. > > Zero 400 ns Compression: 7255 ns. > > Zero 412 ns Compression: 7016 ns. > > Zero 411 ns Compression: 7035 ns. > > Zero 413 ns Compression: 6994 ns. > > Zero 399 ns Compression: 7024 ns. > > Zero 416 ns Compression: 7053 ns. > > Zero 405 ns Compression: 7041 ns. > > > > Indeed, zero-detection is faster than compression. > > > > However during our profiling for the live_migration thread (after reverted this patch), > > we noticed zero-detection cost lots of CPU: > > > > 12.01% kqemu qemu-system-x86_64 [.] buffer_zero_sse2 ◆ > > 7.60% kqemu qemu-system-x86_64 [.] ram_bytes_total ▒ > > 6.56% kqemu qemu-system-x86_64 [.] qemu_event_set ▒ > > 5.61% kqemu qemu-system-x86_64 [.] qemu_put_qemu_file ▒ > > 5.00% kqemu qemu-system-x86_64 [.] __ring_put ▒ > > 4.89% kqemu [kernel.kallsyms] [k] copy_user_enhanced_fast_string ▒ > > 4.71% kqemu qemu-system-x86_64 [.] compress_thread_data_done ▒ > > 3.63% kqemu qemu-system-x86_64 [.] ring_is_full ▒ > > 2.89% kqemu qemu-system-x86_64 [.] __ring_is_full ▒ > > 2.68% kqemu qemu-system-x86_64 [.] threads_submit_request_prepare ▒ > > 2.60% kqemu qemu-system-x86_64 [.] ring_mp_get ▒ > > 2.25% kqemu qemu-system-x86_64 [.] ring_get ▒ > > 1.96% kqemu libc-2.12.so [.] memcpy > > > > After this patch, the workload is moved to the worker thread, is it > > acceptable? > > It depends on your point of view. If you have spare / idle CPUs on the host, > then moving workload to a thread is ok, despite the CPU cost of compression > in that thread being much higher what what was replaced, since you won't be > taking CPU resources away from other contending workloads. It depends on teh VM as well; if the VM is mostly non-zero, the zero checks happen and are over head (although if the pages are non-zero then the zero check will mostly happen much faster unless you're unlucky and the non-zero byte is the last one on the page). > I'd venture to suggest though that we should probably *not* be optimizing for > the case of idle CPUs on the host. More realistic is to expect that the host > CPUs are near fully committed to work, and thus the (default) goal should be > to minimize CPU overhead for the host as a whole. From this POV, zero-page > detection is better than compression due to > x10 better speed. Note that this is only happening if compression is enabled. > Given the CPU overheads of compression, I think it has fairly narrow use > in migration in general when considering hosts are often highly committed > on CPU. Also, this compression series was originally written by Intel for the case where there's a compression accelerator hardware (that I've never found to try); in that case I guess it saves that CPU overhead. Dave > Regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
