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Re: [Qemu-devel] [PATCH 06/12] migration: do not detect zero page for co
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From: |
Dr. David Alan Gilbert |
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Subject: |
Re: [Qemu-devel] [PATCH 06/12] migration: do not detect zero page for compression |
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Date: |
Mon, 16 Jul 2018 19:58:00 +0100 |
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User-agent: |
Mutt/1.10.0 (2018-05-17) |
* 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.
>
> >
> > > 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 / address@hidden / Manchester, UK