Re: [dpdk-dev] [PATCH v6 2/2] eal: add asynchronous request API to DPDK IPC

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

> From: Thomas Monjalon [mailto:thomas@monjalon.net]
> Sent: Wednesday, March 28, 2018 8:30 AM
> To: Tan, Jianfeng <jianfeng.tan@intel.com>
> Cc: Burakov, Anatoly <anatoly.burakov@intel.com>; dev@dpdk.org; Ananyev,
> Konstantin <konstantin.ananyev@intel.com>; Van Haaren, Harry
> <harry.van.haaren@intel.com>
> Subject: Re: [dpdk-dev] [PATCH v6 2/2] eal: add asynchronous request API to
> DPDK IPC
> 
> 28/03/2018 04:08, Tan, Jianfeng:
> > Hi Thomas ,
> >
> > From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > > 27/03/2018 15:59, Anatoly Burakov:
> > > > Under the hood, we create a separate thread to deal with replies to
> > > > asynchronous requests, that will just wait to be notified by the
> > > > main thread, or woken up on a timer.
> > >
> > > I really don't like that a library is creating a thread.
> > > We don't even know where the thread is created (which core).
> > > Can it be a rte_service? or in the interrupt thread?
> >
> > Agree that we'd better not adding so many threads in a library.
> >
> > I was considering to merge all the threads into the interrupt thread,
> however, we don't have an interrupt thread in freebsd. Further, we don't
> implement alarm API in freebsd. That's why I tend to current implementation,
> and optimize it later.
> 
> I would prefer we improve the current code now instead of polluting more
> with more uncontrolled threads.
> 
> > For rte_service, it may be not a good idea to reply on it as it needs
> explicit API calls to setup.
> 
> I don't see the issue of the explicit API.
> The IPC is a new service.

Although I do like to see new services, if we want to enable "core" dpdk functionality with Services, we need a proper designed solution for that. Service cores is not intended for "occasional" work - there is no method to block and sleep on a specific service until work becomes available, so this would imply a busy-polling. Using a service (hence busy polling) for rte_malloc()-based memory mapping requests is inefficient, and total overkill :)

For this patch I suggest to use some blocking-read capable mechanism.

The above said, in the longer term it would be good to have a design that allows new file-descriptors to be added to a "dpdk core" thread, which performs occasional lengthy work if the FD has data available.