Hi there,
I wondered if anyone had come across this particular problem regarding linux
scheduling, or rather what appears to be a forced descheduling effect.
I'm running on standard vanilla Ubuntu 17-10 using kernel 4.13.0-36-generic.
Local Timer interrupts are therefore enabled....
I'm running a dual CPU Xeon E5-2623v4 system. I have cpu 2 on the first NUMA
node (CPU 0) isolated for DPDK receive. I have an Intel X550 card attached to
NUMA 0.
What I'm doing is running my DPDK receive thread on the isolated core (2) and
changing the scheduling for this thread to SCHED_FIFO and priority 98.
Most of the time this works really well. However, I'm running this DPDK thread
inside a larger application - there are probably 40 threads inside this process
at default priority.
What I'm seeing is, when the application is under load, the DPDK receive thread
is forcibly descheduled (observed with pidstat -p <PID> -w and seeing the
non-voluntary counts spike ) and the core appears to go idle, sometimes for up
to 1400uS.
This is obviously a problem....
Running "perf" to sample activity on this isolated core only, I see the
following entries.
0.90% swapper [kernel.kallsyms] [k] cpu_idle_poll
0.60% lcore-slave-2 [kernel.kallsyms] [k] clear_page_erms
i.e - it has gone idle and 1.5% of the processing time has gone elsewhere -
which ties in pretty well with my ~1400uS deschedule observation.
In normal operation I do not see this effect.
I've checked the code - it appears to go idle in the middle of some AVX2 data
processing code - there are no system calls taken, it just goes idle.
Does anyone have any ideas ?
Many thanks
Terry
