[dpdk-dev] Service lcores and Application lcores

[Van Haaren, Harry]

> From: Jerin Jacob [mailto:jerin.jacob at caviumnetworks.com]
> Sent: Friday, June 30, 2017 2:21 PM
> To: Van Haaren, Harry <harry.van.haaren at intel.com>
> Cc: Richardson, Bruce <bruce.richardson at intel.com>; dev at dpdk.org; thomas at monjalon.net;
> Wiles, Keith <keith.wiles at intel.com>
> Subject: Re: Service lcores and Application lcores
> 
> -----Original Message-----
> > Date: Fri, 30 Jun 2017 13:08:26 +0000
> > From: "Van Haaren, Harry" <harry.van.haaren at intel.com>
> > To: Jerin Jacob <jerin.jacob at caviumnetworks.com>
> > CC: "Richardson, Bruce" <bruce.richardson at intel.com>, "dev at dpdk.org"
> >  <dev at dpdk.org>, "thomas at monjalon.net" <thomas at monjalon.net>, "Wiles,
> >  Keith" <keith.wiles at intel.com>
> > Subject: RE: Service lcores and Application lcores
> >
> > > From: Jerin Jacob [mailto:jerin.jacob at caviumnetworks.com]
> > > Sent: Friday, June 30, 2017 1:52 PM
> > > To: Van Haaren, Harry <harry.van.haaren at intel.com>
> > > Cc: Richardson, Bruce <bruce.richardson at intel.com>; dev at dpdk.org; thomas at monjalon.net;
> > > Wiles, Keith <keith.wiles at intel.com>
> > > Subject: Re: Service lcores and Application lcores
> > >
> > > -----Original Message-----
> > > > Date: Fri, 30 Jun 2017 10:00:18 +0000
> > > > From: "Van Haaren, Harry" <harry.van.haaren at intel.com>
> > > > To: Jerin Jacob <jerin.jacob at caviumnetworks.com>, "Richardson, Bruce"
> > > >  <bruce.richardson at intel.com>
> > > > CC: "dev at dpdk.org" <dev at dpdk.org>, "thomas at monjalon.net"
> > > >  <thomas at monjalon.net>, "Wiles, Keith" <keith.wiles at intel.com>
> > > > Subject: RE: Service lcores and Application lcores
> >
> > <snip previous non-related items>
> >
> > > > I don't think providing a remote-launch API is actually beneficial. Remote-launching
> a
> > > single service
> > > > is equivalent to adding that lcore as a service-core, and mapping it to just that
> single
> > > service.
> > > > The advantage of adding it as a service core, is future-proofing for if more
> services
> > > need to be added
> > > > to that core in future, and statistics of the service core infrastructure. A
> convenience
> > > API could be
> > > > provided to perform the core_add(), service_start(), enable_on_service() and
> > > core_start() APIs in one.
> > > >
> > > > Also, the remote_launch API doesn't solve the original problem - what if an
> application
> > > lcore wishes
> > > > to run one iteration of a service "manually". The remote_launch style API does not
> solve
> > > this problem.
> > >
> > > Agree with problem statement. But, remote_launch() operates on lcores not on
> > > not necessary on 1:1 mapped physical cores.
> > >
> > > By introducing "rte_service_iterate", We are creating a parallel infrastructure to
> > > run the service on non DPDK service lcores aka normal lcores.
> > > Is this really required? Is there  any real advantage for
> > > application not use builtin service lcore infrastructure, rather than iterating over
> > > "rte_service_iterate" and run on normal lcores. If we really want to mux
> > > a physical core to N lcore, EAL already provides that in the form of threads.
> > >
> > > I think, providing too many parallel options for the same use case may be
> > > a overkill.
> > >
> > > Just my 2c.
> >
> >
> > The use-case that the rte_service_iterate() caters for is one where the application
> > wishes to run a service on an "ordinary app lcore", together with an application
> workload.
> >
> > For example, the eventdev-scheduler and one worker can be run on the same lcore. If the
> schedule() running thread *must* be a service lcore, we would not be able to also use that
> lcore as an application worker core.
> >
> > That was my motivation for adding this API, I do agree with you above; it is a second
> "parallel" method to run a service. I think there's enough value in enabling the use-case
> as per example above to add it.
> >
> >
> > Do you see enough value in the use-case above to add the API?
> 
> The above use case can be realized like --lcores='(0-1)@1'(Two lcore on
> an physical core). I believe, application writers never want to write a
> code based on specific number of cores available in the system. If they
> do then they will be stuck on running on another environment and too
> many combination to address.

Good point.

> For me it complicates service lcore usage. But someone think, it will useful then
> I don't have strong objection.

We can easily add APIs later - and removing them isn't so easy. +1 from me leave it out for now, and we can see about adding it for 17.11 if the need arises.

Thanks for your input, I'll spin a v3 without the rte_service_iterate() function, and that should be it then!