linux-kernel - Re: [PATCH 34/35] async: use workqueue for worker pool

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Date:	Tue, 29 Jun 2010 11:08:11 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Tejun Heo <htejun@...il.com>
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	torvalds@...ux-foundation.org, mingo@...e.hu,
	linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, rusty@...tcorp.com.au,
	cl@...ux-foundation.org, dhowells@...hat.com, oleg@...hat.com,
	axboe@...nel.dk, dwalker@...eaurora.org, stefanr@...6.in-berlin.de,
	florian@...kler.org, andi@...stfloor.org, mst@...hat.com,
	randy.dunlap@...cle.com, Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH 34/35] async: use workqueue for worker pool

On 6/29/2010 10:12 AM, Tejun Heo wrote:
> On 06/29/2010 06:59 PM, Tejun Heo wrote:
>    
>> Hello, Arjan.
>>
>> On 06/29/2010 06:40 PM, Arjan van de Ven wrote:
>>      
>>> uh? clearly the assumption is that if I have a 16 CPU machine, and 12
>>> items of work get scheduled,
>>> that we get all 12 running in parallel. All the smarts of cmwq surely
>>> only kick in once you've reached the
>>> "one work item per cpu" threshold ???
>>>        
>> Hmmm... workqueue workers are bound to certain cpu, so if you schedule
>> a work on a specific CPU, it will run there.  Once a cpu gets
>> saturated, the issuing thread will be moved elsewhere.  I don't think
>> it matters to any of the current async users one way or the other,
>> would it?
>>      
> Thinking more about it.  It's now not difficult to add a gcwq for an
> unbound pseudo CPU number and use it as host for workers which can run
> on any CPU.  The automatic concurrency management doesn't make much
> sense for those workers, so @max_active can be used as the explicit
> concurrency throttle.  It's not even gonna take a lot of code but I'm
> just not convinced that there's much benefit in doing that.  So, yeah,
> if necessary, sure, but let's think whether it's gonna be actually
> useful.
>    


the point in general is to get maximum parallelism; with systems getting 
more and more cores, maximum parallelism is
a good design goal.

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