Re: [Qemu-devel] [PATCH 1/1] monitor: increase amount of data for monitor to read

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, May 02, 2017 at 09:34:55AM -0500, Eric Blake wrote:
> On 05/02/2017 08:47 AM, Denis V. Lunev wrote:
> > Right now QMP and HMP monitors read 1 byte at a time from the socket, which
> > is very inefficient. With 100+ VMs on the host this easily reasults in
> 
> s/reasults/results/
> 
> > a lot of unnecessary system calls and CPU usage in the system.
> > 
> > This patch changes the amount of data to read to 4096 bytes, which matches
> > buffer size on the channel level. Fortunately, monitor protocol is
> > synchronous right now thus we should not face side effects in reality.
> 
> Do you have any easy benchmarks or measurements to prove what sort of
> efficiencies we get?  (I believe they exist, but quantifying them never
> hurts)
> 
> > 
> > Signed-off-by: Denis V. Lunev <den@openvz.org>
> > CC: Markus Armbruster <armbru@redhat.com>
> > CC: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > CC: Eric Blake <eblake@redhat.com>
> > ---
> >  monitor.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/monitor.c b/monitor.c
> > index be282ec..00df5d0 100644
> > --- a/monitor.c
> > +++ b/monitor.c
> > @@ -3698,7 +3698,7 @@ static int monitor_can_read(void *opaque)
> >  {
> >      Monitor *mon = opaque;
> >  
> > -    return (mon->suspend_cnt == 0) ? 1 : 0;
> > +    return (mon->suspend_cnt == 0) ? 4096 : 0;
> 
> Is a hard-coded number correct, or should we be asking the channel for
> an actual number?

There's no need - this will cause the chardev code to just do a
gio_channel_read() with a 4096 byte buffer. The chardev backend
impl will then happily return fewer bytes than this - just whatever
happens to be pending. IOW this is just acting as an upper bound
on the amount of data we read at once. So 4k seems reasonable to
me, given the typical size of QMP/HMP command strings.

Regards,
Daniel
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