Re: [PATCH] libata: use single threaded work queue

[Jeff Garzik <jeff@garzik.org>]

>>> It would be nice to just create these threads on-demand,
>>> and destroy them again after periods of dis-use.
>>> Kind of like how Apache does worker threads.
>>
>> Well, that's the same thread pool suggestion that Jeff came up with. And
>> I agree, that's a nicer long term solution (it's also how the per-bdi
>> flushing replacement works). The problem with that appears to be that
>> any suggested patchset for thread pools spiral into certain "but what
>> color should it be?!" death.
>
> Let people complain with code :) libata has two basic needs in this area:
> (1) specifying a thread count other than "1" or "nr-cpus"
> (2) don't start unneeded threads / idle out unused threads

To be even more general,

libata needs a workqueue or thread pool that can

(a) scale up to nr-drives-that-use-pio threads, on demand
(b) scale down to zero threads, with lack of demand

That handles the worst case of each PIO-polling drive needing to sleep 
(thus massively impacting latency, if any other PIO-polling drive must 
wait for a free thread).

That also handles the best case of not needing any threads at all.

	Jeff