On Thu, Apr 6, 2017 at 10:02 PM, Stefan Hajnoczi <stefa...@redhat.com> wrote:
> On Wed, Apr 05, 2017 at 05:27:58PM +0800, jemmy858...@gmail.com wrote:
>> From: Lidong Chen <lidongc...@tencent.com>
>>
>> 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.
>
> bdrv_is_allocated() is supposed to yield back to the event loop if it
> needs to block. If your VNC session is experiencing jitter then it's
> probably because a system call in the bdrv_is_allocated() code path is
> synchronous when it should be asynchronous.
>
> You could try to identify the system call using strace -f -T. In the
> output you'll see the duration of each system call. I guess there is a
> file I/O system call that is taking noticable amounts of time.
yes, i find the reason where bdrv_is_allocated needs to block.
the mainly reason is caused by qemu_co_mutex_lock invoked by
qcow2_co_get_block_status.
qemu_co_mutex_lock(&s->lock);
ret = qcow2_get_cluster_offset(bs, sector_num << 9, &bytes,
&cluster_offset);
qemu_co_mutex_unlock(&s->lock);
other reason is caused by l2_load invoked by
qcow2_get_cluster_offset.
/* load the l2 table in memory */
ret = l2_load(bs, l2_offset, &l2_table);
if (ret < 0) {
return ret;
}
>
> A proper solution is to refactor the synchronous code to make it
> asynchronous. This might require invoking the system call from a
> thread pool worker.
>
yes, i agree with you, but this is a big change.
I will try to find how to optimize this code, maybe need a long time.
this patch is not a perfect solution, but can alleviate the problem.
> Stefan
