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

[858585 jemmy]

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.
>
> Regards,
> Daniel
> --
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