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

[Xiao Guangrong <guangrong.xiao@gmail.com>]

On 07/17/2018 02:58 AM, Dr. David Alan Gilbert wrote:
> * Xiao Guangrong (guangrong.xiao@gmail.com) wrote:
>>
>>
>> On 06/29/2018 05:42 PM, Dr. David Alan Gilbert wrote:
>>> * Xiao Guangrong (guangrong.xiao@gmail.com) 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@gmail.com wrote:
>>>>>> From: Xiao Guangrong <xiaoguangrong@tencent.com>
>>>>>>
>>>>>> 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.... :(