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

[Xiao Guangrong]

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

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? :)