[PATCH 14/16] conf: Expose QEMU's main loop object

[Daniel P. Berrangé]

On Tue, Jun 07, 2022 at 10:06:16AM +0200, Michal Prívozník wrote:
> On 6/6/22 13:40, Daniel P. Berrangé wrote:
> > On Thu, Jun 02, 2022 at 01:54:39PM +0200, Peter Krempa wrote:
> >> On Thu, Jun 02, 2022 at 09:18:04 +0200, Michal Privoznik wrote:
> >>> As of v7.0.0-877-g70ac26b9e5 QEMU exposes its main event loop as
> >>> an QMP object. In the very next commit (v7.0.0-878-g71ad4713cc)
> >>> it was extended for thread-pool-min and thread-pool-max
> >>> attributes. Expose them under new <mainloop/> element.
> >>>
> >>> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
> >>> ---
> >>>  docs/formatdomain.rst                         |  5 ++
> >>>  src/conf/domain_conf.c                        | 57 +++++++++++++++++++
> >>>  src/conf/domain_conf.h                        |  8 +++
> >>>  src/conf/schemas/domaincommon.rng             | 15 +++++
> >>>  src/conf/virconftypes.h                       |  2 +
> >>>  .../iothreads-ids-pool-sizes.xml              |  1 +
> >>>  6 files changed, 88 insertions(+)
> >>>
> >>> diff --git a/docs/formatdomain.rst b/docs/formatdomain.rst
> >>> index de085f616a..5e0019c574 100644
> >>> --- a/docs/formatdomain.rst
> >>> +++ b/docs/formatdomain.rst
> >>
> >> [...]
> >>
> >>> @@ -699,6 +700,10 @@ host/guest with many LUNs. :since:`Since 1.2.8 (QEMU only)`
> >>>     The element has two optional attributes ``pool_min`` and ``pool_max`` which
> >>>     allow setting lower and upper boundary for number of worker threads for
> >>>     given IOThread. :since:`Since 8.4.0`
> >>> +``mainloop``
> >>> +   The optional ``mainloop`` element contains two optional attributes
> >>> +   ``pool_min`` and ``pool_max`` which allow setting lower and upper boundary
> >>> +   for number of worker threads for the main event loop. :since:`Since 8.4.0`
> >>
> >> I don't think we should use the qemu term "main loop" here. In general
> >> we refer to the qemu process as emulator, including stuff like
> >> emulatorpin and similar.
> > 
> > It isn't even especially part of the QEMU main loop IIUC.
> > 
> > Rather it is setting up a pool of threads that are used
> > for serving I/O, when no specific I/O thread is configurd
> > for the guest.
> > 
> > Perhaps it can be '<defaultiothread/>' or something along
> > those lines, to make it clear it is an I/O related tunable.
> 
> Right, I'd rather avoid putting it as an attribute to <emulator/> since
> we have a whole section dedicated to performance tuning. Not to mention
> <emulator/> lives under <devices/> and I don't think we put performance
> related knobs there. I didn't object to Peter's suggestion because I
> don't have better idea.
> 
> So are you suggesting that <defaultiothread/> would be at the same level
> as <iothreads/> and <iothreadids/> or it would be nested somewhere?
> 
> <domain>
>   <name/>
>   <iothreads>4</iothreads>
>   <iothreadids>
>     <iothread id="1" thread_pool_min="2" thread_pool_max="8"/>
>     <iothread id="2"/>
>   <iothreadids>
>   <defaultiothread thread_pool_min="8" thread_pool_max="8"/>
>   <devices/>
> </domain>

Yeah, that's pretty much what I was implying.


With regards,
Daniel
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