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 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):

Can you show the matching trace with the patch in?

Dave

> 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
> 
> I guess its atomic operations cost CPU resource and check-before-lock is
> a common tech, i think it shouldn't have side effect, right? :)
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK