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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Thu, Jun 28, 2018 at 05:12:39PM +0800, Xiao Guangrong 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                                                                                                                                                                           ◆
> >   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?
> 
> It depends on your point of view. If you have spare / idle CPUs on the host,
> then moving workload to a thread is ok, despite the CPU cost of compression
> in that thread being much higher what what was replaced, since you won't be
> taking CPU resources away from other contending workloads.

It depends on teh VM as well; if the VM is mostly non-zero, the zero
checks happen and are over head (although if the pages are non-zero then
the zero check will mostly happen much faster unless you're unlucky and
the non-zero byte is the last one on the page).

> I'd venture to suggest though that we should probably *not* be optimizing for
> the case of idle CPUs on the host. More realistic is to expect that the host
> CPUs are near fully committed to work, and thus the (default) goal should be
> to minimize CPU overhead for the host as a whole. From this POV, zero-page
> detection is better than compression due to > x10 better speed.

Note that this is only happening if compression is enabled.

> Given the CPU overheads of compression, I think it has fairly narrow use
> in migration in general when considering hosts are often highly committed
> on CPU.

Also, this compression series was originally written by Intel for the
case where there's a compression accelerator hardware (that I've never
found to try); in that case I guess it saves that CPU overhead.

Dave

> Regards,
> Daniel
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK