Re: [RFC PATCH] iothread: add set_iothread_poll_* commands

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Zhenyu Ye (yezhenyu2@huawei.com) wrote:
> 
> 
> On 2019/10/23 23:19, Stefan Hajnoczi wrote:
> > On Tue, Oct 22, 2019 at 04:12:03PM +0800, yezhenyu (A) wrote:
> >> Since qemu2.9, QEMU added three AioContext poll parameters to struct
> >> IOThread: poll_max_ns, poll_grow and poll_shrink. These properties are
> >> used to control iothread polling time.
> >>
> >> However, there isn't properly hmp commands to adjust them when the VM is
> >> alive. It's useful to adjust them online when observing the impact of
> >> different property value on performance.
> >>
> >> This patch add three hmp commands to adjust iothread poll-* properties
> >> for special iothread:
> >>
> >> set_iothread_poll_max_ns: set the maximum polling time in ns;
> >> set_iothread_poll_grow: set how many ns will be added to polling time;
> >> set_iothread_poll_shrink: set how many ns will be removed from polling
> >> time.
> >>
> >> Signed-off-by: Zhenyu Ye <yezhenyu2@huawei.com>
> >> ---
> >> hmp-commands.hx | 42 ++++++++++++++++++++
> >> hmp.c | 30 +++++++++++++++
> >> hmp.h | 3 ++
> >> include/sysemu/iothread.h | 6 +++
> >> iothread.c | 80 +++++++++++++++++++++++++++++++++++++++
> >> qapi/misc.json | 23 +++++++++++
> >> 6 files changed, 184 insertions(+)
> > 
> > poll-max-ns, poll-grow, poll-shrink are properties of IOThread objects.
> > They can already be modified at runtime using:
> > 
> >   $ qemu -object iothread,id=iothread1
> >   (qemu) qom-set /objects/iothread1 poll-max-ns 100000
> > 
> > I think there is no need for a patch.
> > 
> > Stefan
> > 
> 
> Thanks for your review. I have considered using the `qom-set` command to modify
> IOThread object's properties, however, this command is not friendly to primary
> users. The help info for this command is only:
> 
>     qom-set path property value -- set QOM property
> 
> It's almost impossible to get the correct `path` parameter for primary user.

Is this just a matter of documenting how to do it?

It sounds like there's no need for a new QMP command though;  if you
want an easier HMP command I'd probably still take it (because HMP is ok
at having things for convenience) - but not if it turns out that just
adding a paragraph of documentation is enough.

Dave

> This patch provides a more convenient and easy-use hmp&qmp interface to modify
> these IOThread properties. I think this patch still has a little value.
> 
> And I can implement this patch compactly by reusing your code.
> 
> Waiting for your reply.
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK