kern.sched.quantum: Creepy, sadistic scheduler

Peter pmc at
Wed Apr 4 20:13:27 UTC 2018

Andriy Gapon wrote:
> On 04/04/2018 03:52, Peter wrote:
>> Lets run an I/O-active task, e.g, postgres VACUUM that would
>> continuousely read from big files (while doing compute as well [1]):
> Not everyone has a postgres server and a suitable database.
> Could you please devise a test scenario that demonstrates the problem and that
> anyone could run?


and maybe nobody anymore has such old system that is CPU-bound instead 
of IO-bound. I'd rather think about reproducing it on my IvyBridge.

I know for sure that it is *not* specifically dependent on postgres.
What I posted was the case when an endless-loop piglet starves a 
postgres VACUUM - and there we see a very pronounced effect of almost
factor 100.
When I first clearly discovered it (after a long time of belly-feeling 
that something behaves strange), it was postgres pg_dump (which does 
compression, i.e. CPU-bound) as the piglet starving an bacula-fd
backup that would scan the filesystem.

So, there is a general rule: we have one process that is a CPU-hog, and 
another process that does periodic I/O (but also *some* compute). and 
-important!- nothing else.

If we understand the logic of the scheduler, that information should 
already suit for some logical verification *eg* - but I will see if I 
get it reprocuved on the IvyBridge machine and/or see if I get a 
testcase together. May take a while.


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