* Xiao Guangrong (guangrong.xiao@xxxxxxxxx) 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 ◆ Interesting; what host are you running on? Some hosts have support for the faster buffer_zero_ss4/avx2 > 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? > > > > > From compression rate POV of course zero page algo wins since it > > contains no data (but only a flag). > > > > Yes it is. The compressed zero page is 45 bytes that is small enough i think. So the compression is ~20x slow and 10x the size; not a great improvement! However, the tricky thing is that in the case of a guest which is mostly non-zero, this patch would save that time used by zero detection, so it would be faster. > Hmm, if you do not like, how about move detecting zero page to the work thread? That would be interesting to try. Dave > Thanks! -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
