Re: [PATCH v4] sched/fair: Prefer fully idle cores for NOHZ balancing

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 8/4/26 11:39 PM, Andrea Righi wrote:
> Hi Shrikanth,
> 

>> Just Curious, making nohz_full=<except first core> yeilds similar numbers?
> 
> Yes, provided that the housekeeping core is also excluded from the workload.
> 
> I tested this booting with:
> 
>    nohz_full=1-175,177-351
> 
> This leaves CPUs 0 and 176 for housekeeping. I excluded that core from the
> workload and ran 87 OpenMP tasks on the remaining node-0 cores (1 task per core,
> excluding the housekeeping one):
> 
>   $ env OMP_NUM_THREADS=87 \
>         OMP_DYNAMIC=false \
>         OPENBLAS_LOOPS=10 \
>         OPENBLAS_PARAM_M=16384 \
>         OPENBLAS_PARAM_N=16384 \
>         OPENBLAS_PARAM_K=16384 \
>         numactl -C 1-87,177-263 --membind=0 \
>         ./benchmark/sgemm.goto 1 1 1
> 
> Results:
> 
>      unpatched           : 5.246 TFLOP/s
>      unpatched+nohz_full : 6.953 TFLOP/s
>      patched             : 6.861 TFLOP/s
>      patched+nohz_full   : 6.984 TFLOP/s
> 
> So, nohz_full seems to prevent the problematic ILB wakeups and can produce
> similar results for this CPU-bound workload. However, it shouldn't be considered
> a sobstiute for the ILB fix, since it requires explicit partitioning and
> reserved housekeeping CPUs. Full dyntick also enables context tracking on the
> isolated CPUs, adding kernel entry/exit overhead.
> 

That's good to know. Thanks for running them.
I put "just curious", knowing nohz_full is not a solution.