Re: [dpdk-dev] [PATCH v1 1/6] build: increase default of max lcores to 512

[Thomas Monjalon <thomas@monjalon.net>]

10/09/2021 10:06, David Marchand:
> On Fri, Sep 10, 2021 at 9:54 AM Bruce Richardson
> <bruce.richardson@intel.com> wrote:
> >
> > On Fri, Sep 10, 2021 at 08:51:04AM +0200, David Marchand wrote:
> > > On Thu, Sep 9, 2021 at 4:38 PM Bruce Richardson
> > > <bruce.richardson@intel.com> wrote:
> > > >
> > > > On Thu, Sep 09, 2021 at 02:45:06PM +0100, David Hunt wrote:
> > > > > Modern processors are coming with an ever increasing number of cores,
> > > > > and 128 does not seem like a sensible max limit any more, especially
> > > > > when you consider multi-socket systems with Hyper-Threading enabled.
> > > > >
> > > > > This patch increases max_lcores default from 128 to 512.
> > > > >
> > > > > Signed-off-by: David Hunt <david.hunt@intel.com>
> > >
> > > Why should we need this?
> > >
> > > --lcores makes it possible to pin 128 lcores to any physical core on
> > > your system.
> > > And for applications that have their own thread management, they can
> > > pin thread, then use rte_thread_register.
> > >
> > > Do you have applications that require more than 128 lcores?
> > >
> > The trouble is that using the --lcores syntax for mapping high core numbers
> > to low lcore ids is much more awkward to use. Every case of DPDK use I've
> > seen uses -c with a coremask, or -l with just giving a few core numbers on
> > it. This simple scheme won't work with core numbers greater than 128, and
> > there are already systems available with more than that number of cores.
> >
> > Apart from the memory footprint issues - which this patch is already making
> > a good start in addressing, why would we not increase the default
> > max_lcores to that seen on real systems?
> 
> The memory footprint is a major issue to me, and reserving all those
> lcores won't be needed in any system.
> We will also have to decide on a "640k ought to be enough" value to
> avoid ABI issue with the next processor that comes out and has more
> than 512 cores.
> 
> Could we wire the -c / -l options to --lcores behavior ?
> It breaks the 1:1 lcore/physical core assumption, but it solves your
> usability issue.

Why would we change existing options while we already have an option
(--lcores) which solves the issue above?
I think the only issue is to educate users.
Is there something to improve in the documentation?