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From: Brian Drummond <brian@shapes.demon.co.uk>
Newsgroups: comp.lang.ada
Subject: Re: Ada 2012 automatic concurrency?
Date: Thu, 27 Dec 2012 11:22:02 +0000 (UTC)
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Date: 2012-12-27T11:22:02+00:00
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On Thu, 27 Dec 2012 09:59:44 +0100, Dirk Heinrichs wrote:

> charleshixsn@earthlink.net wrote:
> 
>> IIUC, there shouldn't be more than about twice as many tasks as cores. 
>> Is this about right?
> 
> Hmm, taking into account that your application isn't alone on your
> system and task switches have their price, too, I think you'd better go
> with "# of CPUs/Cores + 1", like the Gentoo Linux Handbook suggests for
> the level of parallel compilation:
> 
> "A good choice is the number of CPUs (or CPU cores) in your system plus
> one,
> but this guideline isn't always perfect." [1]
> 
> However, I'm still confused about the "+1".

Probably an attempt to ensure there is always one task waiting to do 
useful work when a currently running task suspends. 

I have my doubts about the "+1" too in practice, since it may mean tasks 
migrating between CPUs and trashing their caches.

There may be times when it is better to have "+1" per CPU, and mark each 
task with affinity for the CPU it starts out with. 

In any case, too many tasks means too much inactive state, which 
increases pressure on cache and slows up task swapping. So "+1" may be 
more of an ideal asymptotic target than a practical reality (like Colin 
Chapman's adage about the perfect race car - one that falls apart while 
crossing the finish line in first place).

It would be interesting to see some numbers though.

- Brian