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Re: [Qemu-devel] [PATCH 07/12] migration: hold the lock only if it is re
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH 07/12] migration: hold the lock only if it is really needed |
Date: |
Fri, 13 Jul 2018 18:44:46 +0100 |
User-agent: |
Mutt/1.10.0 (2018-05-17) |
* Xiao Guangrong (address@hidden) 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, address@hidden wrote:
> > > > > From: Xiao Guangrong <address@hidden>
> > > > >
> > > > > 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 <address@hidden>
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK