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Re: [Qemu-devel] [PATCH 06/12] migration: do not detect zero page for co
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH 06/12] migration: do not detect zero page for compression |
Date: |
Fri, 29 Jun 2018 10:54:18 +0100 |
User-agent: |
Mutt/1.10.0 (2018-05-17) |
* Daniel P. Berrangé (address@hidden) 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, 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?
>
> 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 / address@hidden / Manchester, UK
- [Qemu-devel] [PATCH 05/12] migration: show the statistics of compression, (continued)
[Qemu-devel] [PATCH 07/12] migration: hold the lock only if it is really needed, guangrong . xiao, 2018/06/04
[Qemu-devel] [PATCH 08/12] migration: do not flush_compressed_data at the end of each iteration, guangrong . xiao, 2018/06/04
[Qemu-devel] [PATCH 09/12] ring: introduce lockless ring buffer, guangrong . xiao, 2018/06/04