Re: [Qemu-devel] [PATCH v3] migration/block:limit the time used for block migration

[858585 jemmy]

On Wed, Apr 5, 2017 at 6:44 PM, 858585 jemmy <address@hidden> wrote:
> On Wed, Apr 5, 2017 at 5:34 PM, Daniel P. Berrange <address@hidden> wrote:
>> On Wed, Apr 05, 2017 at 05:27:58PM +0800, address@hidden wrote:
>>> From: Lidong Chen <address@hidden>
>>>
>>> when migration with high speed, mig_save_device_bulk invoke
>>> bdrv_is_allocated too frequently, and cause vnc reponse slowly.
>>> this patch limit the time used for bdrv_is_allocated.
>>
>> Can you explain why calling bdrv_is_allocated is impacting VNC performance ?
>>
>
> bdrv_is_allocated is called after qemu_mutex_lock_iothread.
>
>     if (bmds->shared_base) {
>         qemu_mutex_lock_iothread();
>         aio_context_acquire(blk_get_aio_context(bb));
>         /* Skip unallocated sectors; intentionally treats failure as
>          * an allocated sector */
>         while (cur_sector < total_sectors &&
>                !bdrv_is_allocated(blk_bs(bb), cur_sector,
>                                   MAX_IS_ALLOCATED_SEARCH, &nr_sectors)) {
>             cur_sector += nr_sectors;
>         }
>         aio_context_release(blk_get_aio_context(bb));
>         qemu_mutex_unlock_iothread();
>     }
>
> and the main thread is also call qemu_mutex_lock_iothread.
>
> #0  0x00007f107322f264 in __lll_lock_wait () from /lib64/libpthread.so.0
> #1  0x00007f107322a508 in _L_lock_854 () from /lib64/libpthread.so.0
> #2  0x00007f107322a3d7 in pthread_mutex_lock () from /lib64/libpthread.so.0
> #3  0x0000000000949ecb in qemu_mutex_lock (mutex=0xfc51a0) at
> util/qemu-thread-posix.c:60
> #4  0x0000000000459e58 in qemu_mutex_lock_iothread () at 
> /root/qemu/cpus.c:1516
> #5  0x0000000000945322 in os_host_main_loop_wait (timeout=28911939) at
> util/main-loop.c:258
> #6  0x00000000009453f2 in main_loop_wait (nonblocking=0) at 
> util/main-loop.c:517
> #7  0x00000000005c76b4 in main_loop () at vl.c:1898
> #8  0x00000000005ceb77 in main (argc=49, argv=0x7fff921182b8,
> envp=0x7fff92118448) at vl.c:4709
>
>> Migration is running in a background thread, so shouldn't be impacting the
>> main thread which handles VNC, unless the block layer is perhaps acquiring
>> the global qemu lock ? I wouldn't expect such a lock to be held for just
>> the bdrv_is_allocated call though.
>>

I'm not sure it's safe to remove qemu_mutex_lock_iothread. i will
analyze and test it later.
this patch is simple, and can solve the problem now.

>> Regards,
>> Daniel
>> --
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