lcores clarification

[Stephen Hemminger]

On Tue, 18 Oct 2022 22:06:04 +0000
Juan Pablo L. <jpablolorenzetti at hotmail.com> wrote:

> Hellos guys, I have a doubt that I have not been able to clarify with the docs, so maybe you can help me please ...
> 
> I have a process that I want to run on a set of CPUs (performing the same task) but I want to run all of them at the same time, so my question is:
> 
> if I configure something like "--lcores='2@(5-7)'" that will make my lcore with id 2 run simultaneously (at the same time in parallel) on CPUs 5,6,7 or it will run my lcore id 2 only one time at any given time on either CPU 5,6 or 7 ?
> 
> would it be better if I configure different lcore id for the same and assign it to individual CPUs ? , e.g: "--lcores='2 at 5,3 at 6,4 at 7'" even though lcore id 2,3,4 are really the same job ?
> 
> I think my question is just a matter of what is the best way to configure EAL to accomplish what I want, that is to run the same process on all CPU available concurrently, even though, I believe, option 2 seems to be the obvious way I just want to make sure I am not doing something that DPDK could do for me already ....
> 
> A bonus question if its not too much, since lcores are really pthreads (in linux) and I have isolated the CPUs from the kernel scheduler, my guess is that if I have different lcores (performing the same or different tasks) running on the same CPU DPDK will be doing the scheduling right ? ..
> 
> thank you very much!!!!
> 

If you have multiple threads running on same lcore, you better not use locks or rings,
or mbufs, or most of the DPDK. Because these features all work like:

  while (resource is busy)
     spin wait

So if one thread has a resource and gets preempted. When another thread runs
and tries to acquire the same resource; the thread will spin until scheduler
decides to run the other thread.

This is not unique to DPDK, the same problem is described in the pthread_spinlock
man page.