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

[Dr. David Alan Gilbert]

* 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

>   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.

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

That would be interesting to try.

Dave

> Thanks!
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK