Re: [PATCH] mm: Move readahead limit outside of readahead, and advisory syscalls

[Rafael Aquini <aquini@redhat.com>]

On Mon, Jul 25, 2016 at 01:47:32PM -0700, Andrew Morton wrote:
> On Mon, 25 Jul 2016 10:39:25 -0400 Kyle Walker <kwalker@redhat.com> wrote:
> 
> > Java workloads using the MappedByteBuffer library result in the fadvise()
> > and madvise() syscalls being used extensively. Following recent readahead
> > limiting alterations, such as 600e19af ("mm: use only per-device readahead
> > limit") and 6d2be915 ("mm/readahead.c: fix readahead failure for
> > memoryless NUMA nodes and limit readahead pages"), application performance
> > suffers in instances where small readahead is configured.
> 
> Can this suffering be quantified please?
> 
> > By moving this limit outside of the syscall codepaths, the syscalls are
> > able to advise an inordinately large amount of readahead when desired.
> > With a cap being imposed based on the half of NR_INACTIVE_FILE and
> > NR_FREE_PAGES. In essence, allowing performance tuning efforts to define a
> > small readahead limit, but then benefiting from large sequential readahead
> > values selectively.
> > 
> > ...
> >
> > --- a/mm/readahead.c
> > +++ b/mm/readahead.c
> > @@ -211,7 +211,9 @@ int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
> >  	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);
> > +	nr_to_read = min(nr_to_read, (global_page_state(NR_INACTIVE_FILE) +
> > +				     (global_page_state(NR_FREE_PAGES)) / 2));
> > +
> >  	while (nr_to_read) {
> >  		int err;
> >  
> > @@ -484,6 +486,7 @@ void page_cache_sync_readahead(struct address_space *mapping,
> >  
> >  	/* be dumb */
> >  	if (filp && (filp->f_mode & FMODE_RANDOM)) {
> > +		req_size = min(req_size, inode_to_bdi(mapping->host)->ra_pages);
> >  		force_page_cache_readahead(mapping, filp, offset, req_size);
> >  		return;
> >  	}
> 
> Linus probably has opinions ;)
>

IIRC one of the issues Linus had with previous attempts was because 
they were utilizing/bringing back a node-memory state based heuristic. 

Since Kyle patch is using a global state counter for that matter,
I think that issue condition might now be sorted out.

-- Rafael

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