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

Christoph Hellwig
2021-01-13T14:50:41Z

> +/* 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.