RE: [PATCH v3 0/3] Introduce per-task latency_nice for scheduler hints

[David Laight <David.Laight@ACULAB.COM>]

From: chris hyser
> Sent: 18 February 2020 23:00
...
> All, I was asked to take a look at the original latency_nice patchset.
> First, to clarify objectives, Oracle is not
> interested in trading throughput for latency.
> What we found is that the DB has specific tasks which do very little but
> need to do this as absolutely quickly as possible, ie extreme latency
> sensitivity. Second, the key to latency reduction
> in the task wakeup path seems to be limiting variations of "idle cpu" search.
> The latter particularly interests me as an example of "platform size
> based latency" which I believe to be important given all the varying size
> VMs and containers.

From my experiments there are a few things that seem to affect latency
of waking up real time (sched fifo) tasks on a normal kernel:

1) The time taken for the (intel x86) cpu to wakeup from monitor/mwait.
   If the cpu is allowed to enter deeper sleep states this can take 900us.
   Any changes to this are system-wide not process specific.

2) If the cpu an RT process last ran on (ie the one it is woken on) is
   running in kernel, the process switch won't happen until cond_reshed()
   is called.
   On my system the code to flush the display frame buffer takes 3.3ms.
   Compiling a kernel with CONFIG_PREEMPT=y will reduce this.

3) If a hardware interrupt happens just after the process is woken
   then you have to wait until it finishes and any 'softint' work
   that is scheduled on the same cpu finishes.
   The ethernet driver transmit completions an receive ring filling
   can easily take 1ms.
   Booting with 'threadirq' might help this.

4) If you need to acquire a lock/futex then you need to allow for the
   process that holds it being delayed by a hardware interrupt (etc).
   So even if the lock is only held for a few instructions it can take
   a long time to acquire.
   (I need to change some linked lists to arrays indexed by an atomically
   incremented global index.)

FWIW I can't imagine how a database can have anything that is that
latency sensitive.
We are doing lots of channels of audio processing and have a lot of work
to do within 10ms to avoid audible errors.

	David

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