Sleeping in Work_queue threads

Sleeping in Work_queue threads

[Rajat Jain]

Hi,

I understand that there is a default worker thread ("event") to which
most of the drivers submit work (for bottom half processing).

Now since this thread is catering to all of the drivers; if a driver's
code sleeps, then aren't the rest of the drivers penalised?

I mean the rest of the drivers want to do their work but the thread is
now sleeping since one of the driver has issued a sleeping call?

Wouldn't a seperate worker thread for each driver be a better design?

Thanks & Sincere Regards,

Rajat Jain
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Re: Sleeping in Work_queue threads

[borasahin]

> I understand that there is a default worker thread ("event") to which
> most of the drivers submit work (for bottom half processing).
>
> Now since this thread is catering to all of the drivers; if a driver's
> code sleeps, then aren't the rest of the drivers penalised?

Yes...

> I mean the rest of the drivers want to do their work but the thread is
> now sleeping since one of the driver has issued a sleeping call?
>
> Wouldn't a seperate worker thread for each driver be a better design?

I think the worker thread is there to make driver writer's job easier. Say you 
need a bottom half but it is not sleep for a long time. Using a centralised 
worker thread is the most convenient one. But as you say if you need to sleep 
longer then you start to disrupt other driver's worker jobs. It's time to 
create a seperate worker thread. 

Another point of view -> you categorize worker threads: shorter sleep times 
and longer sleep times. You dont need to create a worker thread for each of 
the work jobs. Instead you use one for shorter sleep times and one for each 
of longer sleep times. So you use system resources more efficiently. I think 
most of worker thread jobs is fit into shorter sleep times category.

--
Bora SAHIN
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Re: Sleeping in Work_queue threads

[Jim Cromie]

borasahin@justnic.com wrote:
>> I understand that there is a default worker thread ("event") to which
>> most of the drivers submit work (for bottom half processing).
>>
>> Now since this thread is catering to all of the drivers; if a driver's
>> code sleeps, then aren't the rest of the drivers penalised?
>>     
>
> Yes...
>
>   
>> I mean the rest of the drivers want to do their work but the thread is
>> now sleeping since one of the driver has issued a sleeping call?
>>
>> Wouldn't a seperate worker thread for each driver be a better design?
>>     
>
> I think the worker thread is there to make driver writer's job easier. Say you 
> need a bottom half but it is not sleep for a long time. Using a centralised 
> worker thread is the most convenient one. But as you say if you need to sleep 
> longer then you start to disrupt other driver's worker jobs. It's time to 
> create a seperate worker thread. 
>
> Another point of view -> you categorize worker threads: shorter sleep times 
> and longer sleep times. You dont need to create a worker thread for each of 
> the work jobs. Instead you use one for shorter sleep times and one for each 
> of longer sleep times. So you use system resources more efficiently. I think 
> most of worker thread jobs is fit into shorter sleep times category.
>
>   

so hypothetically, if there were such a set of worker threads,
how would I pick the right thread ?
or rather, how do I measure/characterize the sleep-time ?
(worst case, average, etc)

does the Latency Tracer  (see: http://lwn.net/Articles/97811/ )
make this possible/easy ?

and while we're on the topic, whats the average length of the event 
work-queue ?
and the enque/deque rate ?
would 1 long queue, and 1 short cover 99%

> --
> Bora SAHIN
>
> --
> Kernelnewbies: Help each other learn about the Linux kernel.
> Archive:       http://mail.nl.linux.org/kernelnewbies/
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>
>
>   

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Re: Sleeping in Work_queue threads

[borasahin]

> > I think the worker thread is there to make driver writer's job easier.
> > Say you need a bottom half but it is not sleep for a long time. Using a
> > centralised worker thread is the most convenient one. But as you say if
> > you need to sleep longer then you start to disrupt other driver's worker
> > jobs. It's time to create a seperate worker thread.
> >
> > Another point of view -> you categorize worker threads: shorter sleep
> > times and longer sleep times. You dont need to create a worker thread for
> > each of the work jobs. Instead you use one for shorter sleep times and
> > one for each of longer sleep times. So you use system resources more
> > efficiently. I think most of worker thread jobs is fit into shorter sleep
> > times category.
>
> so hypothetically, if there were such a set of worker threads,
> how would I pick the right thread ?
> or rather, how do I measure/characterize the sleep-time ?
> (worst case, average, etc)
>
> does the Latency Tracer  (see: http://lwn.net/Articles/97811/ )
> make this possible/easy ?

In case if I fail to express it, I was trying to explain the rationale behind 
Linux worker thread model. There is a common thread. If it doesnt satisfy 
your requirements(maybe your worker thread are processor intensive or maybe 
you do longer sleeps or maybe you are in a situation where tighter latency is 
needed) you create on your owns. So basically you have two choice.

Otherwise, IMHO such a mechanism is not required. Timers are implemented that 
way. But some clear categorization is exist. I dont think worker threads 
gives such a way. In order to be able to do it quantitiy should be more so 
doing it gives some benefit. But even that may not be applicable. Because 
except timers, tasklets and workqueues delays execution. But how much time is 
more blur. Besides there are ways of doing it in the workqueue model: 
schedule_delayed_work -> http://lxr.linux.no/source/kernel/workqueue.c#L396

--
Bora SAHIN

> and while we're on the topic, whats the average length of the event
> work-queue ?
> and the enque/deque rate ?
> would 1 long queue, and 1 short cover 99%
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