linux-kernel - Re: [PATCH] Replace completions with semaphores

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Date:	Wed, 16 Apr 2008 18:07:24 +0200
From:	Ingo Oeser <ioe-lkml@...eria.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Matthew Wilcox <matthew@....cx>, Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Bart Van Assche <bart.vanassche@...il.com>,
	Roland Dreier <rdreier@...co.com>, Ingo Molnar <mingo@...e.hu>,
	Daniel Walker <dwalker@...sta.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Replace completions with semaphores

On Tuesday 15 April 2008, Linus Torvalds wrote:
> Would it make sense to use completions for countable events too? Yeah. In 
> fact, we have some things that really would like to do counting, both in 
> the sense of "wait for <n> events to all complete" _and_ in the sense of 
> "allow up to <n> events to be outstanding". Both of which could be done as 
> a counting function (just make "complete" increment the counter, and then 
> make "wait for <n> events" initialize it to negative, while "allow <n>
> outstanding events" would be a positive counter, and make 
> "wait_for_completion()" basically be a "decrement and wait until it 

Hmm, why not introduce a provide_slots(), 
wait_for_slot() and free_slot() API?

provide_slots() :=  initialize a resource with N available slots
wait_for_slot() :=  wait for a slot in countable resource to be free
free_slot() :=  make a slot available

That would:
- fit the use case nicely
- could be optimized for countable resources
  with upper limits
- allow to measure slot utilization
- ...
- is not called semaphore :-)

And isn't this the same problem (called scheduling) as:
- keeping N cpus busy
- keeping N (disk) spindles busy
- keeping N nic transmit queues busy
- ...

???

But maybe I oversimplify here :-)


Best Regards

Ingo Oeser
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