Linux scheduler, overscheduling performance, threads

[Ingo Molnar]

On Thu, 20 Jan 2000, Brian Hurt wrote:

> Following this thread off and on since it's inception, and being a Java
> programmer myself, can I offer some observations?
> 
> Thousands of threads in a program is not unreasonable.  If you may want to
> take full advantage of a 128 CPU machine, for example, you need _at_
> _least_ 128 threads.  If your threads spend most of their time blocking,
> you need even more threads, you need to overschedule, to make sure you
> generally have enough threads not blocking to make sure CPUs aren't going
> to waste.  Unfortunately, due to vagaries of the system, you will have
> points when most of the threads become runnable at once.

i believe you are confusing '2000 threads created and happily waiting
blocked to do something' and the '2000 threads running' case. I
specifically included a quick benchmark showing 20 thousand threads being
around on my system and not impacting Linux scheduler performance the
slightest. What i say is that more than nr_cpus*2 _running_ (ie. currently
waiting for the CPU, not some other resource) threads are bad.

-- mingo


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