[dpdk-dev] [RFC PATCH 0/7] support multi-phtread per lcore

Walukiewicz, Miroslaw Miroslaw.Walukiewicz at intel.com
Tue Dec 23 10:19:54 CET 2014



> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Stephen
> Hemminger
> Sent: Monday, December 22, 2014 7:29 PM
> To: Richardson, Bruce
> Cc: dev at dpdk.org
> Subject: Re: [dpdk-dev] [RFC PATCH 0/7] support multi-phtread per lcore
> 
> On Mon, 22 Dec 2014 09:46:03 +0000
> Bruce Richardson <bruce.richardson at intel.com> wrote:
> 
> > On Mon, Dec 22, 2014 at 01:51:27AM +0000, Liang, Cunming wrote:
> > > ...
> > > > I'm conflicted on this one. However, I think far more applications would
> be
> > > > broken
> > > > to start having to use thread_id in place of an lcore_id than would be
> broken
> > > > by having the lcore_id no longer actually correspond to a core.
> > > > I'm actually struggling to come up with a large number of scenarios
> where it's
> > > > important to an app to determine the cpu it's running on, compared to
> the large
> > > > number of cases where you need to have a data-structure per thread.
> In DPDK
> > > > libs
> > > > alone, you see this assumption that lcore_id == thread_id a large
> number of
> > > > times.
> > > >
> > > > Despite the slight logical inconsistency, I think it's better to avoid
> introducing
> > > > a thread-id and continue having lcore_id representing a unique thread.
> > > >
> > > > /Bruce
> > >
> > > Ok, I understand it.
> > > I list the implicit meaning if using lcore_id representing the unique thread.
> > > 1). When lcore_id less than RTE_MAX_LCORE, it still represents the logical
> core id.
> > > 2). When lcore_id large equal than RTE_MAX_LCORE, it represents an
> unique id for thread.
> > > 3). Most of APIs(except rte_lcore_id()) in rte_lcore.h suggest to be used
> only in CASE 1)
> > > 4). rte_lcore_id() can be used in CASE 2), but the return value no matter
> represent a logical core id.
> > >
> > > If most of us feel it's acceptable, I'll prepare for the RFC v2 base on this
> conclusion.
> > >
> > > /Cunming
> >
> > Sorry, I don't like that suggestion either, as having lcore_id values greater
> > than RTE_MAX_LCORE is terrible, as how will people know how to
> dimension arrays
> > to be indexes by lcore id? Given the choice, if we are not going to just use
> > lcore_id as a generic thread id, which is always between 0 and
> RTE_MAX_LCORE
> > we can look to define a new thread_id variable to hold that. However, it
> should
> > have a bounded range.
> > From an ease-of-porting perspective, I still think that the simplest option is
> to
> > use the existing lcore_id and accept the fact that it's now a thread id rather
> > than an actual physical lcore. Question is, is would that cause us lots of
> issues
> > in the future?
> >
> > /Bruce
> 
> The current rte_lcore_id() has different meaning the thread. Your proposal
> will
> break code that uses lcore_id to do per-cpu statistics and the lcore_config
> code in the samples.
> q
It depends on application context and how application treats rte_lcore_id() core. When number of the threads will not exceed the number of cores (let's say old-fashioned DPDK application) all stuff like per-cpu statistics will work correctly. 

When we treat threads on cores as ordinary threads as we introducing the special function rte_pthread_create() - the meaning of rte_lcore_id() changes to indicate 
 thread number what is correct under new assumptions and new application model.

I do not  want to limit DPDK design  to only per-cpu application. There is much more application models that could be supported using DPDK. 
Current per-cpu approach is only a subset of the possible applications.

Maybe we should indicate something like CONFIG_RTE_PTHREAD_ENABLE to change a meaning of rte_lcore_id() and introducing rte_pthread_create() family. 

Mirek



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