Re: [Qemu-devel] [PATCH 06/12] migration: do not detect zero page for compression

[Xiao Guangrong]

On 07/17/2018 02:58 AM, Dr. David Alan Gilbert wrote:
> * Xiao Guangrong (address@hidden) wrote:
> > On 06/29/2018 05:42 PM, Dr. David Alan Gilbert wrote:
> > > * Xiao Guangrong (address@hidden) 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, address@hidden wrote:
> > > > > > From: Xiao Guangrong <address@hidden>
> > > > > >
> > > > > > 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.

Er, it is not easy to update glibc in the production env.... :(