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From: 858585 jemmy <jemmy858585@gmail.com>
Date: Fri, 7 Apr 2017 09:30:33 +0800
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Subject: Re: [Qemu-devel] [PATCH v3] migration/block:limit the time used for
 block migration
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To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: qemu-devel@nongnu.org, Fam Zheng <famz@redhat.com>, quintela@redhat.com, dgilbert@redhat.com, qemu-block@nongnu.org, Lidong Chen <lidongchen@tencent.com>

On Thu, Apr 6, 2017 at 10:02 PM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> On Wed, Apr 05, 2017 at 05:27:58PM +0800, jemmy858585@gmail.com wrote:
>> From: Lidong Chen <lidongchen@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