linux-kernel - Re: Overview of concurrency managed workqueue

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Message-ID: <20100618083622.6c117f3e@schatten.dmk.lab>
Date:	Fri, 18 Jun 2010 08:36:22 +0200
From:	Florian Mickler <florian@...kler.org>
To:	Daniel Walker <dwalker@...eaurora.org>
Cc:	Tejun Heo <tj@...nel.org>, mingo@...e.hu, awalls@...ix.net,
	linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, rusty@...tcorp.com.au,
	cl@...ux-foundation.org, dhowells@...hat.com,
	arjan@...ux.intel.com, johannes@...solutions.net, oleg@...hat.com,
	axboe@...nel.dk
Subject: Re: Overview of concurrency managed workqueue

On Thu, 17 Jun 2010 11:03:49 -0700
Daniel Walker <dwalker@...eaurora.org> wrote:

> 
> I don't agree with your analogy here .. It's more like you have two
> items in the vending machine item A and item B. Tejun is saying he likes
> item A, his friends like item A so that must mean item B is not
> interesting so he removes it (without knowing how many people want it). 
> So what happens? People use another vending machine. (Linux is the
> vending machine) ..

fair enough. let's stop the analogies here, i'm already sorry I
started with it. :)

> >From my perspective this is like using Linux only for throughput which
> is what Tejun is maximizing. In addition Tejun is blowing up the
> alternative which is to prioritize the work items and sticking you with
> strictly a throughput based system.

No, the fact that multiple workers work on a workqueue decreases
latency as well...  (worst case latency would be the same, if there were
a minimum limit for worker threads per queue. I'm not shure if this is
implemented.. )

> 
> Do you thing maybe it's possible that the work items aren't all created
> equal? Maybe one item _is_ more important than another one. Maybe on a
> given system Tejun's workqueues runs a 1000 useless pointless work items
> before he gets to the valuable one .. Yet the user is powerless to
> dictate what is or is not important.

you have a point here. I think the priority should go with the
work-item though. Or do you think that is not a good solution? 

one priority-inheritance would be to increase the priority of all
work-items queued before the "valuable" work item. another way is to
just have enough worker-threads so that all work-items are executed
as fast as possible (if necessary summoning new workers).  a third is to
have a separate worker-pool and workqueue for high-priority work. and
i'm guessing there are more solutions...

> > Once you have the priority in the driver you could pass it to the
> > workqueue subsystem (i.e. set the priority of the work) and the worker
> > could then assume the priority of its work.
> > 
> > The tricky part is probably to pass the priority from the userspace
> > thread to the kernel?
> 
> Threads are designed to have priorities tho, and threads are pervasive
> throughout Linux .. It seems like setting a priorities to drivers would
> be like re-inventing the wheel ..

No, no. Not setting priorities to drivers. What I wanted to get at, is
that the one who shedules the work has to/can decide what priority that
work should run as. I.e. the priority has to go with the work. 

Because you are upping the priority of a thread not for the thread's
sake but for the work the thread is going to execute/executing? 

> 
> If you were to say make the driver use kthreads instead of workqueues,
> then you could set the priority of the kthreads .. However, you said
> this isn't part of the ABI and so your back to your original argument
> which is that you shouldn't be setting priorities in the first place.

I don't follow. What I thought about was, that the "workqueue"
interface is defined in workqueue.h. 

There is no "increase_priority_of_workqueue()" or
"increase_work_priority()" at the moment in the interface description. 

Fumbling at the threads is using _implementation knowledge_ that should
be hidden by the interface. 
I'm not saying this is a binary true/false kind of "fact", just one
view point.

Also I agree that some ability to prioritize work
items has to be enabled. And using the scheduler for that is the only
sane alternative. But I think the workqueue-implementation should do
it, so it has the freedom to dispatch threads at will. 

> 
> Daniel
> 
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