Re: [RFCv2] service core concept

["Van Haaren, Harry" <harry.van.haaren@intel.com>]

> From: Ananyev, Konstantin
> Sent: Tuesday, June 6, 2017 4:29 PM
> Subject: RE: [dpdk-dev] [RFCv2] service core concept
> 
> 
> > From: Richardson, Bruce
> > Sent: Tuesday, June 6, 2017 3:54 PM
> >
> > On Tue, Jun 06, 2017 at 11:25:57AM +0100, Van Haaren, Harry wrote:
> > > > From: Ananyev, Konstantin
> > > > Sent: Saturday, June 3, 2017 11:23 AM
> > >
> > > <snip>

<snip other discussion>

> > >
> > > There are a number of options here, each with its own merit:
> > >
> > > A) Services/cores config in EAL
> > > Benefit is that service functionality can be transparent to the application. Negative is
> that the complexity is in EAL.
> > >
> > > B) Application configures services/cores
> > > Benefit is no added EAL complexity. Negative is that application code has to configure
> cores (duplicated per application).
> > >
> > >
> > > To answer this question, I think we need to estimate how many applications would benefit
> from EAL integration and balance that against
> > the "complexity cost" of doing so. I do like the simplicity of option (B), however if there
> is significant value in total transparency to the
> > application I think (A) is the better choice.
> > >
> > >
> > > Input on A) or B) welcomed! -Harry
> >
> > I'm definitely in favour of having it in EAL. The whole reason for doing
> > this work is to make it easy for applications to dedicate cores to
> > background tasks - including applications written before this
> > functionality was added. By merging this into EAL, we can have
> > transparency in the app, as we can have the service cores completely in
> > the background, and the app can call rte_eal_mp_remote_launch() exactly
> > as before, without unexpected failures. If we move this externally, the
> > app needs to be reworked to take account of that fact, and call new,
> > service-core aware, launch functions instead.
> 
> Not sure I understood you here:
> If the app don' plan to use any cores for services, it for sure will be able to call
> rte_eal_mp_remote_launch() as before (no services running case).

Correct - EAL behavior remains unchanged if --service-cores=0xf is not passed


> From other side, if the app would like to use services - it would need to specify
> which service it wants to run, and for each service provide a coremask, even if
> EAL already allocates service cores for it.

See next paragraph


> Or are you talking about the when EAL allocates service cores, and then
> PMDs themselves (or EAL again) register their services on that cores?

EAL could provide sane default behavior. For example, round-robin services over available service-cores. Multithread-capable services can be registered on all service cores. Its not a perfect solution for all service-to-core mapping problems, but I'd guess about 80% of cases would be covered: using a single service with a single service core dedicated to it :)


> That's probably possible, but how PMD would know which service core(s) it allowed to use?

The PMD shouldn't be deciding - EAL for basic sanity config, or Application for advanced usage.


> Things might get over-complicated here - in theory there could be multiple PMDs,
> each of them can have more than one service, running on multiple sets of cores, etc.

True - the NxM service:core mapping possibility can be huge - the API allows the application the flexibility if that flexibility is really required. If the flexibility is not required, the round-robin 1:1 service:core EAL scheme should cover it?

-Harry