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From: Frederic Weisbecker on 28 Jun 2010 19:00
On Mon, Jun 28, 2010 at 11:04:22PM +0200, Tejun Heo wrote:
> Replace private worker pool with system_long_wq.


It appeared to me that async is deemed to parallelize as much as
possible, to probe devices faster on boot for example, while cmwq
seems to do the opposite: trying to execute in batches as much as
possible, and fork when a work goes to sleep voluntarily.

That said I haven't checked that deeply so it's fairly possible
I missed something obvious :)

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From: Tejun Heo on 29 Jun 2010 03:30
Hello,

On 06/29/2010 12:55 AM, Frederic Weisbecker wrote:
> On Mon, Jun 28, 2010 at 11:04:22PM +0200, Tejun Heo wrote:
>> Replace private worker pool with system_long_wq.
>
> It appeared to me that async is deemed to parallelize as much as
> possible, to probe devices faster on boot for example, while cmwq
> seems to do the opposite: trying to execute in batches as much as
> possible, and fork when a work goes to sleep voluntarily.

Yeah, well, that's kind of the whole point of cmwq. It would try to
minimize the number of used workers but the provided concurrency will
still be enough. No async probe will be stalled due to lack of
execution context and the timings should be about the same between the
original async implemetnation and cmwq based one.

Thanks.

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From: Frederic Weisbecker on 29 Jun 2010 08:20
On Tue, Jun 29, 2010 at 09:25:12AM +0200, Tejun Heo wrote:
> Hello,
>
> On 06/29/2010 12:55 AM, Frederic Weisbecker wrote:
> > On Mon, Jun 28, 2010 at 11:04:22PM +0200, Tejun Heo wrote:
> >> Replace private worker pool with system_long_wq.
> >
> > It appeared to me that async is deemed to parallelize as much as
> > possible, to probe devices faster on boot for example, while cmwq
> > seems to do the opposite: trying to execute in batches as much as
> > possible, and fork when a work goes to sleep voluntarily.
>
> Yeah, well, that's kind of the whole point of cmwq. It would try to
> minimize the number of used workers but the provided concurrency will
> still be enough. No async probe will be stalled due to lack of
> execution context and the timings should be about the same between the
> original async implemetnation and cmwq based one.
>
> Thanks.


Right. I just don't know what is supposed to be slow on boot that needs
to use async. Is that because reading some ports is slow or because we
need to do something and wait for some times to get the result.

If there is a question of slow ports to probe, then cmwq wouldn't seem the
right thing here, as it only forks when we go to sleep.

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From: Tejun Heo on 29 Jun 2010 11:50
Hello,

On 06/29/2010 02:18 PM, Frederic Weisbecker wrote:
>> Yeah, well, that's kind of the whole point of cmwq. It would try to
>> minimize the number of used workers but the provided concurrency will
>> still be enough. No async probe will be stalled due to lack of
>> execution context and the timings should be about the same between the
>> original async implemetnation and cmwq based one.
>
> Right. I just don't know what is supposed to be slow on boot that
> needs to use async. Is that because reading some ports is slow or
> because we need to do something and wait for some times to get the
> result.

It's things like ATA bus resetting and probing. They're usually
composed of short CPU activities and rather long sleeps.

> If there is a question of slow ports to probe, then cmwq wouldn't seem the
> right thing here, as it only forks when we go to sleep.

I lost you here. If something during boot has to burn cpu cycles
(which it shouldn't, really), it has to burn cpu cycles and having
multiple concurent threads won't help anything. If something doesn't
burn cpu cycles but takes long, it gotta sleep and cmwq will start a
new thread immediately. So, can you please elaborate why cmwq would
be problematic?

Thanks.

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From: Tejun Heo on 29 Jun 2010 12:00
Hello,

On 06/29/2010 05:52 PM, Frederic Weisbecker wrote:
>>> If there is a question of slow ports to probe, then cmwq wouldn't seem the
>>> right thing here, as it only forks when we go to sleep.
>>
>> I lost you here. If something during boot has to burn cpu cycles
>> (which it shouldn't, really), it has to burn cpu cycles and having
>> multiple concurent threads won't help anything.
>
> It would on SMP.

Oh, I see. Parallel cpu hogs. We don't have such users for async and
I think using padata would be the right solution for those situations.

Thanks.

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