* Xiao Guangrong (guangrong.xiao@xxxxxxxxx) wrote: > > > On 06/29/2018 05:42 PM, Dr. David Alan Gilbert wrote: > > * 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 > > The host is: > > model name : Intel(R) Xeon(R) Gold 6142 CPU @ 2.60GHz > ... > flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi > mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts > rep_good nopl xtopology nonstop_tsc cpuid aperfmperf tsc_known_freq pni pclmulqdq dtes64 monitor > ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic movbe popcnt > tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l3 > cdp_l3 intel_ppin intel_pt mba tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust bmi1 > hle avx2 smep bmi2 erms invpcid rtm cqm mpx rdt_a avx512f avx512dq rdseed adx smap clflushopt > clwb avx512cd avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total > cqm_mbm_local dtherm ida arat pln pts hwp hwp_act_window hwp_epp hwp_pkg_req pku ospke > > I checked and noticed "CONFIG_AVX2_OPT" has not been enabled, maybe is due to too old glib/gcc > version: > gcc version 4.4.6 20110731 (Red Hat 4.4.6-4) (GCC) > glibc.x86_64 2.12 Yes, that's pretty old (RHEL6 ?) - I think you should get AVX2 in RHEL7. > > > > > > 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. > > Yes, indeed. It would be good to benchmark the performance difference for a guest with mostly non-zero pages; you should see a useful improvement. Dave > > > > > Hmm, if you do not like, how about move detecting zero page to the work thread? > > > > That would be interesting to try. > > > > Okay, i will try it then. :) > -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
