[GSoC][PATCH v2 3/6] qemu_domainjob: `maxQueuedJobs` added to `qemuDomainJobPrivate`

[Erik Skultety]

On Wed, Aug 19, 2020 at 05:04:58PM +0200, Michal Privoznik wrote:
> On 8/19/20 2:00 PM, Erik Skultety wrote:
> > On Wed, Aug 19, 2020 at 01:45:37PM +0200, Michal Privoznik wrote:
> > > On 8/18/20 10:48 AM, Erik Skultety wrote:
> > > > On Mon, Aug 17, 2020 at 10:37:18AM +0530, Prathamesh Chavan wrote:
> > > > > Reference to `maxQueuedJobs` required us to access
> > > > > config of the qemu-driver. And creating its copy in
> > > > > the `qemuDomainJob` helped us access the variable
> > > > > without referencing the driver's config.
> > > > >
> > > > > Signed-off-by: Prathamesh Chavan <pc44800 at gmail.com>
> > > > > ---
> > > > >    src/qemu/qemu_domain.c    |  5 ++++-
> > > > >    src/qemu/qemu_domainjob.c | 13 +++++++------
> > > > >    src/qemu/qemu_domainjob.h |  4 +++-
> > > > >    3 files changed, 14 insertions(+), 8 deletions(-)
> > > > >
> > >
> > >
> > > > Looking at this again, I don't think we want to move any of the jobs_queued or
> > > > maxQueuedJobs attributes to the job structure, I'm sorry I didn't spot this
> > > > right away in the previous revision of the series, I take responsibility for
> > > > it.
> > > > The job structure represents a single job, so from the design POV,
> > > > referencing both jobs_queued (which is really tied to a domain) and
> > > > maxQueuedJobs (which is a global driver setting) from within the job structure
> > > > simply doesn't feel right. Instead, I think qemuDomainObjBeginJobInternal will
> > > > simply have to be split in parts specific to QEMU that have access to the
> > > > driver and hypervisor-agnostic bits that can be moved to src/hypervisor.
> > >
> > > Actually, I think maxQueuedJobs is job specific and not a global driver
> > > setting. I mean, I can imagine other drivers benefiting from it too.
> > > It's true that we currently don't allow specifying different values for
> > > different domains and only apply the setting from qemu.conf, but in the
> > > future we might make this available in domain XML somehow [1] and thus be
> > > per domain. Moreover, I can imagine users wanting to set the limit for
> > > volumes too [2].
> >
> > I can relate to the reasoning, but I still don't think the Job structure is the
> > right place, like I said, it holds everything that a single job needs, from
> > that POV a single job doesn't really need to do anything with that value, it's
> > just it happens to be a very convenient place right now, I say let's try
> > finding a different location for the attribute where it makes more sense.
>
> Sure, we can probably find different location, but question then is how
> usable the APIs would be? My idea of a good API is like this:
>
> job = initJob(parameters);  /* alternatively, initJob(&job, parameters); */
>
> beginJob(&job, type);
>
> /* do something useful */
>
> endJob(&job);
>
> If we'd have maxQueuedJobs separate, then we would have to pass it in
> beginJob(), right?

Naturally. In a standard OOP design, such information would come from the
object calling the beginJob() method, in our case this would most likely
translate to the domain object over which we're executing a job. We'd therefore
need the domain object as well.

I see what you mean, but a usable API is as important as logically structured
data the API takes.

Erik