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

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH v3] migration/block:limit the time used for block migration
Date: Thu, 6 Apr 2017 15:02:55 +0100
User-agent: Mutt/1.8.0 (2017-02-23)

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.

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.

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.


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