linux-kernel - Re: [PATCH 2/2] block: loop: avoiding too many pending per work I/O

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Date:	Tue, 5 May 2015 09:59:58 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	"Justin M. Forbes" <jforbes@...oraproject.org>,
	Jeff Moyer <jmoyer@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	"v4.0" <stable@...r.kernel.org>
Subject: Re: [PATCH 2/2] block: loop: avoiding too many pending per work I/O

On Tue, May 05, 2015 at 07:49:55PM +0800, Ming Lei wrote:
...
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 3dc1598..1bee523 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -725,7 +725,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
>  		goto out_putf;
>  	error = -ENOMEM;
>  	lo->wq = alloc_workqueue("kloopd%d",
> -			WQ_MEM_RECLAIM | WQ_HIGHPRI | WQ_UNBOUND, 0,
> +			WQ_MEM_RECLAIM | WQ_HIGHPRI | WQ_UNBOUND, 16,

It's a bit weird to hard code this to 16 as this effectively becomes a
hidden bottleneck for concurrency.  For cases where 16 isn't a good
value, hunting down what's going on can be painful as it's not visible
anywhere.  I still think the right knob to control concurrency is
nr_requests for the loop device.  You said that for linear IOs, it's
better to have higher nr_requests than concurrency but can you
elaborate why?

Thanks.

-- 
tejun
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