Date:	Wed, 19 Aug 2009 14:23:21 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Mark Lord <liml@....ca>
Cc:	Jeff Garzik <jeff@...zik.org>, linux-ide@...r.kernel.org,
	linux-kernel@...r.kernel.org, htejun@...il.com,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH] libata: use single threaded work queue

On Wed, Aug 19 2009, Mark Lord wrote:
> Jens Axboe wrote:
>> On Wed, Aug 19 2009, Jeff Garzik wrote:
>>> On 08/19/2009 07:25 AM, Jens Axboe wrote:
>>>> Hi,
>>>>
>>>> On boxes with lots of CPUs, we have so many kernel threads it's not
>>>> funny. The basic problem is that create_workqueue() creates a per-cpu
>>>> thread, where we could easily get by with a single thread for a lot of
>>>> cases.
>>>>
>>>> One such case appears to be ata_wq. You want at most one per pio drive,
>>>> not one per CPU. I'd suggest just dropping it to a single threaded wq.
>>>>
>>>> Signed-off-by: Jens Axboe<jens.axboe@...cle.com>
>>>>
>>>> diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
>>>> index 072ba5e..0d78628 100644
>>>> --- a/drivers/ata/libata-core.c
>>>> +++ b/drivers/ata/libata-core.c
>>>> @@ -6580,7 +6580,7 @@ static int __init ata_init(void)
>>>>   {
>>>>   	ata_parse_force_param();
>>>>
>>>> -	ata_wq = create_workqueue("ata");
>>>> +	ata_wq = create_singlethread_workqueue("ata");
>>>>   	if (!ata_wq)
>>>>   		goto free_force_tbl;
>>>
>>> I agree with one-thread-per-cpu is too much, in these modern 
>>> multi-core  times, but one thread is too little.  You have 
>>> essentially re-created  simplex DMA -- blocking and waiting such that 
>>> one drive out of ~4 can be  used at any one time.
>>>
>>> ata_pio_task() is in a workqueue so that it can sleep and/or spend a  
>>> long time polling ATA registers.  That means an active task can   
>>> definitely starve all other tasks in the workqueue, if only one 
>>> thread  is available.  If starvation occurs, it will potentially 
>>> pause the  unrelated task for several seconds.
>>>
>>> The proposed patch actually expands an existing problem -- 
>>> uniprocessor  case, where there is only one workqueue thread.  For 
>>> the reasons  outlined above, we actually want multiple threads even 
>>> in the UP case.  If you have more than one PIO device, latency is 
>>> bloody awful, with  occasional multi-second "hiccups" as one PIO 
>>> devices waits for another.  It's an ugly wart that users DO 
>>> occasionally complain about; luckily  most users have at most one PIO 
>>> polled device.
>>>
>>> It would be nice if we could replace this workqueue with a thread 
>>> pool,  where thread count inside the pool ranges from zero to $N 
>>> depending on  level of thread pool activity.  Our common case in 
>>> libata would be  _zero_ threads, if so...
>>
>> That would be ideal, N essentially be:
>>
>>         N = min(nr_cpus, nr_drives_that_need_pio);
> ..
>
> No, that would leave just a single thread again for UP.

So one thread per ap would be better, then.

> 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.

I'll try and work up a simple create_threadpool() implementation, we can
literally cut away hundreds of threads with that.

-- 
Jens Axboe

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