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

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Jan 13, 2021 at 03:49:32PM +0100, Christoph Hellwig wrote:
> > +/* 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?

Initially I assigned it to the buftarg code on the grounds that this is
how you'd estimate the level of parallelism available through the data
device buftarg.

I don't really care where it goes, though.  xfs_pwork_guess_threads
would be fine too.

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

Hm, where would we put it?  One of the xfs sysctls?  And would we be
able to resize the background blockgc workqueue size too?

--D