* high-priority RT CPU-hog applications and kernel threads
@ 2020-10-14 16:37 Chris Friesen
0 siblings, 0 replies; only message in thread
From: Chris Friesen @ 2020-10-14 16:37 UTC (permalink / raw)
To: linux-rt-users
Hi,
I'm on the CentOS 7 RT kernel but I'm hoping the general principles
still apply.
I've got a system with a number of CPUs set aside for housekeeping and
others dedicated to low-latency applications. We have "irqaffinity",
"rcu_nocbs", "nohz_full", and "isolcpus" set up to try to isolate the
application CPUs as much as possible, with RCU threads affined to the
housekeeping CPUs where possible. (rcuc/N is still on the application
CPUs.) We have CONFIG_RCU_BOOST=y and CONFIG_RCU_KTHREAD_PRIO=2.
One packet-processing application wants to run in a busy-loop pulling
packets off queues and shoving them onto other queues. No actual system
calls that context-switch into the kernel, only calls to get a timestamp
which are handled by the vDSO. Testing has shown better jitter if we
run this higher priority than all the kernel threads, but I wanted to
make sure this was kosher since the various tuning guides seem to
recommend against running high-RT-priority CPU hogs.
Is it okay to run the application higher-priority than all the RCU
threads since the bulk of the work is offloaded to the housekeeping
CPUs? Would we need "rcu_nocb_poll" to run the application
higher-priority than rcuc/N?
Is it okay to run the application higher-priority than ktimersoftd/N
given that the application doesn't use timers and nothing else should be
running on that CPU?
Is it okay run the application higher-priority than the irq threads or
is that a moot point as long as we affine all the irq threads to the
housekeeping CPUs?
Is there anything else I'm missing? Basically as much as possible I
just want this CPU to be entirely dedicated to the application, with
nothing else running on it. Most of the tuning guides want the kernel
threads higher-priority than the application though, and I'd like to
avoid that if possible.
Thanks,
Chris
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