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

[David Hunt <david.hunt@intel.com>]

On 10/9/2021 9:24 AM, Thomas Monjalon wrote:
> 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?
>

Hi all,
I agree that it’s a good idea to switch to using the “--lcrores” option 
for cores above the default, that’s already future proofed.
However, I’m still a little concerned about usability, if our users are 
accustomed to the “-c” and “-l” options, I suggest that we add a warning 
to suggest using the “--lcores” option if any of the cores provided on 
the command line are above RTE_MAX_LCORE. That would help them with the 
solution to using physical cores above 128 (or whatever the compiled 
default is).

Example:

“ERROR: logical core 212 is above the maximum lcore number permitted.
Please use the --lcores option to map lcores onto physical cores, e.g. 
--lcores="(0-3)@(212-215).”

I’ll replace the first patch in the set with a patch that adds the 
additional information in the error message.

Thanks,
Dave.