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

[Xiao Guangrong]

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              
                                                                                
                                                                             ◆
  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!