Re: [PATCH v4] mm: don't cap request size based on read-ahead setting

[Jens Axboe]

On 11/18/2016 11:02 AM, Johannes Weiner wrote:
> On Thu, Nov 17, 2016 at 02:23:10PM -0700, Jens Axboe wrote:
> > We ran into a funky issue, where someone doing 256K buffered reads saw
> > 128K requests at the device level. Turns out it is read-ahead capping
> > the request size, since we use 128K as the default setting. This doesn't
> > make a lot of sense - if someone is issuing 256K reads, they should see
> > 256K reads, regardless of the read-ahead setting, if the underlying
> > device can support a 256K read in a single command.
> >
> > To make matters more confusing, there's an odd interaction with the
> > fadvise hint setting. If we tell the kernel we're doing sequential IO on
> > this file descriptor, we can get twice the read-ahead size. But if we
> > tell the kernel that we are doing random IO, hence disabling read-ahead,
> > we do get nice 256K requests at the lower level. This is because
> > ondemand and forced read-ahead behave differently, with the latter doing
> > the right thing. An application developer will be, rightfully,
> > scratching his head at this point, wondering wtf is going on. A good one
> > will dive into the kernel source, and silently weep.
>
> With the FADV_RANDOM part of the changelog updated, this looks good to
> me. Just a few nitpicks below.
>
> > This patch introduces a bdi hint, io_pages. This is the soft max IO size
> > for the lower level, I've hooked it up to the bdev settings here.
> > Read-ahead is modified to issue the maximum of the user request size,
> > and the read-ahead max size, but capped to the max request size on the
> > device side. The latter is done to avoid reading ahead too much, if the
> > application asks for a huge read. With this patch, the kernel behaves
> > like the application expects.
> >
> > Signed-off-by: Jens Axboe <axboe@xxxxxx>
>
> > @@ -207,12 +207,17 @@ int __do_page_cache_readahead(struct address_space
> > *mapping, struct file *filp,
> >   * memory at once.
> >   */
> >  int force_page_cache_readahead(struct address_space *mapping, struct file
> > *filp,
>
> Linewrap (but you already knew that ;))

Yeah, dunno wtf happened there...

> > -		pgoff_t offset, unsigned long nr_to_read)
> > +		               pgoff_t offset, unsigned long nr_to_read)
> >  {
> > +	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
> > +	struct file_ra_state *ra = &filp->f_ra;
> > +	unsigned long max_pages;
> > +
> >  	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
> >  		return -EINVAL;
> >
> > -	nr_to_read = min(nr_to_read, inode_to_bdi(mapping->host)->ra_pages);
> > +	max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
> > +	nr_to_read = min(nr_to_read, max_pages);
>
> It would be useful to have the comment on not capping below optimal IO
> size from ondemand_readahead() here as well.

Good idea, I'll copy/paste the comment from the ondemand part.

> > @@ -369,10 +374,18 @@ ondemand_readahead(struct address_space *mapping,
> >  		   bool hit_readahead_marker, pgoff_t offset,
> >  		   unsigned long req_size)
> >  {
> > -	unsigned long max = ra->ra_pages;
> > +	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
> > +	unsigned long max_pages = ra->ra_pages;
> >  	pgoff_t prev_offset;
> >
> >  	/*
> > +	 * If the request exceeds the readahead window, allow the read to
> > +	 * be up to the optimal hardware IO size
> > +	 */
> > +	if (req_size > max_pages && bdi->io_pages > max_pages)
> > +		max_pages = min(req_size, bdi->io_pages);
> > +
> > +	/*
> >  	 * start of file
> >  	 */
> >  	if (!offset)
>
> Please feel free to add:
>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Thanks!

--
Jens Axboe