Re: [Qemu-devel] [PATCH 07/12] migration: hold the lock only if it is really needed

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

* Xiao Guangrong (guangrong.xiao@gmail.com) wrote:
> 
> 
> On 07/11/2018 04:21 PM, Peter Xu wrote:
> > On Thu, Jun 28, 2018 at 05:33:58PM +0800, Xiao Guangrong wrote:
> > > 
> > > 
> > > On 06/19/2018 03:36 PM, Peter Xu wrote:
> > > > On Mon, Jun 04, 2018 at 05:55:15PM +0800, guangrong.xiao@gmail.com wrote:
> > > > > From: Xiao Guangrong <xiaoguangrong@tencent.com>
> > > > > 
> > > > > Try to hold src_page_req_mutex only if the queue is not
> > > > > empty
> > > > 
> > > > Pure question: how much this patch would help?  Basically if you are
> > > > running compression tests then I think it means you are with precopy
> > > > (since postcopy cannot work with compression yet), then here the lock
> > > > has no contention at all.
> > > 
> > > Yes, you are right, however we can observe it is in the top functions
> > > (after revert this patch):
> > > 
> > > Samples: 29K of event 'cycles', Event count (approx.): 22263412260
> > > +   7.99%  kqemu  qemu-system-x86_64       [.] ram_bytes_total
> > > +   6.95%  kqemu  [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
> > > +   6.23%  kqemu  qemu-system-x86_64       [.] qemu_put_qemu_file
> > > +   6.20%  kqemu  qemu-system-x86_64       [.] qemu_event_set
> > > +   5.80%  kqemu  qemu-system-x86_64       [.] __ring_put
> > > +   4.82%  kqemu  qemu-system-x86_64       [.] compress_thread_data_done
> > > +   4.11%  kqemu  qemu-system-x86_64       [.] ring_is_full
> > > +   3.07%  kqemu  qemu-system-x86_64       [.] threads_submit_request_prepare
> > > +   2.83%  kqemu  qemu-system-x86_64       [.] ring_mp_get
> > > +   2.71%  kqemu  qemu-system-x86_64       [.] __ring_is_full
> > > +   2.46%  kqemu  qemu-system-x86_64       [.] buffer_zero_sse2
> > > +   2.40%  kqemu  qemu-system-x86_64       [.] add_to_iovec
> > > +   2.21%  kqemu  qemu-system-x86_64       [.] ring_get
> > > +   1.96%  kqemu  [kernel.kallsyms]        [k] __lock_acquire
> > > +   1.90%  kqemu  libc-2.12.so             [.] memcpy
> > > +   1.55%  kqemu  qemu-system-x86_64       [.] ring_len
> > > +   1.12%  kqemu  libpthread-2.12.so       [.] pthread_mutex_unlock
> > > +   1.11%  kqemu  qemu-system-x86_64       [.] ram_find_and_save_block
> > > +   1.07%  kqemu  qemu-system-x86_64       [.] ram_save_host_page
> > > +   1.04%  kqemu  qemu-system-x86_64       [.] qemu_put_buffer
> > > +   0.97%  kqemu  qemu-system-x86_64       [.] compress_page_with_multi_thread
> > > +   0.96%  kqemu  qemu-system-x86_64       [.] ram_save_target_page
> > > +   0.93%  kqemu  libpthread-2.12.so       [.] pthread_mutex_lock
> > 
> > (sorry to respond late; I was busy with other stuff for the
> >   release...)
> > 
> 
> You're welcome. :)
> 
> > I am trying to find out anything related to unqueue_page() but I
> > failed.  Did I miss anything obvious there?
> > 
> 
> unqueue_page() was not listed here indeed, i think the function
> itself is light enough (a check then directly return) so it
> did not leave a trace here.
> 
> This perf data was got after reverting this patch, i.e, it's
> based on the lockless multithread model, then unqueue_page() is
> the only place using mutex in the main thread.
> 
> And you can see the overload of mutext was gone after applying
> this patch in the mail i replied to Dave.

I got around to playing with this patch and using 'perf top'
to see what was going on.
What I noticed was that without this patch pthread_mutex_unlock and
qemu_mutex_lock_impl were both noticeable; with the patch they'd
pretty mich vanished.  So I think it's worth it.

I ocouldn't honestly see a difference in total CPU usage or
bandwidth; but the migration code is so spiky in usage that
it's difficult to measure anyway.

So,


Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

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