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

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

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                                                                                                                                                                           ◆
   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.

Hmm, if you do not like, how about move detecting zero page to the work thread?

Thanks!