Re: [PATCH 1/7] xfs: increase the default parallelism levels of pwork clients

[Christoph Hellwig <hch@infradead.org>]

> +/* Estimate the amount of parallelism available for a given device. */
> +unsigned int
> +xfs_buftarg_guess_threads(
> +	struct xfs_buftarg	*btp)
> +{
> +	int			iomin;
> +	int			ioopt;
> +
> +	/*
> +	 * The device tells us that it is non-rotational, and we take that to
> +	 * mean there are no moving parts and that the device can handle all
> +	 * the CPUs throwing IO requests at it.
> +	 */
> +	if (blk_queue_nonrot(btp->bt_bdev->bd_disk->queue))
> +		return num_online_cpus();
> +
> +	/*
> +	 * The device has a preferred and minimum IO size that suggest a RAID
> +	 * setup, so infer the number of disks and assume that the parallelism
> +	 * is equal to the disk count.
> +	 */
> +	iomin = bdev_io_min(btp->bt_bdev);
> +	ioopt = bdev_io_opt(btp->bt_bdev);
> +	if (iomin > 0 && ioopt > iomin)
> +		return ioopt / iomin;
> +
> +	/*
> +	 * The device did not indicate that it has any capabilities beyond that
> +	 * of a rotating disk with a single drive head, so we estimate no
> +	 * parallelism at all.
> +	 */
> +	return 1;
> +}

Why is this in xfs_buf.c despite having nothing to do with the buffer
cache?

Also I think we need some sort of manual override in case the guess is
wrong.