Re: RSDL v0.31
From: Nick Piggin
Date: Thu Mar 29 2007 - 03:35:07 EST
David Schwartz wrote:
Bill Davidsen wrote:
I agree for giving a process more than a fair share, but I don't think
"latency" is the best term for what you describe later. If you think of
latency as the time between a process unblocking and the time when it
gets CPU, that is a more traditional interpretation. I'm not really sure
latency and CPU-starved are compatible.
For CPU-starvation, I think 'nice' is always going to be the fix. If you
want a process to get more than its 'fair share' of the CPU, you have to ask
for that. I think the scheduler should be fair by default.
However, cleverness in the scheduler with latency can make things better
without being unfair to anyone. It's perfectly fair for a task that has been
blocked for awhile to pre-empt a CPU-limited task when it unblocks.
What I'm arguing is that if your task is CPU-limited and the scheduler is
fair, that's your fault -- nice it. If your task is suffering from poor
latency, and it's using less than its fair share of the CPU (because it is
not CPU-limited), that is something the scheduler can be smarter about.
Agreed. That's what I've been saying for years (since early 2.6 when we had
all those scheduler troubles and I started nicksched).
Honestly, I have always been against aggressive pre-emption. I think as CPUs
get faster and timeslices get shorter, it makes less and less sense. In many
I think scheduler timeslices actually shouldn't really be getting shorter.
While I found it is quite easy to get good interactivity with a pretty
dumb scheduler and tiny timeslices (at least until load ramps up enough
that the "off-time" for your critical processes builds up too much), I
think we want to aim for large timeslices. CPU caches are still getting
bigger, and I don't think misses are getting cheaper (especially if you
consider multi core). Also, the energy cost of a memory access is much
higher even if hardware or software is able to hide the latency.
--
SUSE Labs, Novell Inc.
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