Li, Tong N wrote:
On Tue, 2007-07-24 at 16:39 -0400, Chris Snook wrote:
Divining the intentions of the administrator is an AI-complete problem and we're
not going to try to solve that in the kernel. An intelligent administrator
could also allocate 50% of each CPU to a resource group containing all the
*other* processes. Then, when the other processes are scheduled out, your
single task will run on whichever CPU is idle. This will very quickly
equilibrate to the scheduling ping-pong you seem to want. The scheduler
deliberately avoids this kind of migration by default because it hurts cache and
TLB performance, so if you want to override this very sane default behavior,
you're going to have to explicitly configure it yourself.
Well, the admin wouldn't specifically ask for 50% of each CPU. He would
just allocate 50% of total CPU time---it's up to the scheduler to
fulfill that. If a task is entitled to one CPU, then it'll stay there
and have no migration. Migration occurs only if there's overload, in
which case I think you agree in your last email that the cache and TLB
impact is not an issue (at least in SMP).
I don't think Chris's scenario has much bearing on your patch. What he wants is
to have a task that will always be running, but can't monopolize either CPU.
This is useful for certain realtime workloads, but as I've said before, realtime
requires explicit resource allocation. I don't think this is very relevant to
SCHED_FAIR balancing.
-- Chris
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