linux-kernel - Re: Work queue questions

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Date:	Fri, 21 Sep 2012 20:26:01 +0100
From:	Deepawali Verma <dverma249@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Work queue questions

Hi Tejun,

I have put the ftrace markers in my code:

     kworker/u:1-21    [000]   110.964895: task_event: MYTASKJOB2381 XStarted
     kworker/u:1-21    [000]   110.964909: task_event: MYTASKJOB2381 Xstopped
     kworker/u:1-21    [000]   110.965137: task_event: MYTASKJOB2382 XStarted
     kworker/u:1-21    [000]   110.965154: task_event: MYTASKJOB2382 Xstopped
     kworker/u:5-3724  [000]   110.965311: task_event: MYTASKJOB2383 XStarted
     kworker/u:5-3724  [000]   110.965325: task_event: MYTASKJOB2383 Xstopped

I have this one big task to whom I divided into small sub tasks. These
are numbered 2381, 2382 and 2383, what was I expecting that task 2381,
2382, 2383 run in parallel. I have put start and stop markers here so
that I can see how this concurrency managed work queue is distributing
the load.

I found that task no 2381 is started first and exited before starting
task 2382 and so on. What I expected that it should start the three
sub tasks in parallel, not one by one.

Where is concurrency here?

Regards,
Deepa



On Fri, Sep 21, 2012 at 7:35 PM, Tejun Heo <tj@...nel.org> wrote:
> Hello,
>
> On Fri, Sep 21, 2012 at 07:30:21PM +0100, Deepawali Verma wrote:
>> Actually I want to make parallelization of one task into three tasks.
>> Therefore I created three single threaded work queues means divide the
>> task into three tasks. You are right that I can use one work queue as
>> well. But when I am doing three times schedule on different work
>> queues, I am seeing only one worker thread is processing the three
>> times schedule though I created three different workqueues and I
>> believe from previous kernel versions that there is one worker thread
>> associated with one queue. If one thread does this task then there is
>> no difference between doing the same task in one thread and using
>> three threads.
>>
>> If we create different work queues, why always one worker thread is
>> processing the all tasks instead I want another two threads also work
>> in parallel?
>
> Well, that was the whole point of concurrency managed workqueue.  You
> don't need to worry about the number of workers.  Concurrency is
> automatically managed.  If you queue three work items on, say,
> system_wq and none of them sleeps, a single worker will execute them
> back to back.  If a work item sleeps, another worker will kick in.
> So, in most cases, there's no need to worry about concurrency - just
> use system_wq.
>
> Thanks.
>
> --
> tejun
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