linux-kernel - Re: [RFC][PATCH] on-demand readahead

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Date:	Thu, 26 Apr 2007 09:16:55 +0800
From:	Fengguang Wu <fengguang.wu@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Andrew Morton <akpm@...l.org>, Oleg Nesterov <oleg@...sign.ru>,
	Steven Pratt <slpratt@...tin.ibm.com>,
	Ram Pai <linuxram@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] on-demand readahead

On Wed, Apr 25, 2007 at 06:08:44PM +0200, Andi Kleen wrote:
> > Yeah, the on-demand readahead can avoid _all_ lookups for small in-cache files.
> 
> How?

In filemap.c:
                if (!page) {
                        page_cache_readahead_adaptive(mapping,
                                        &ra, filp, page,
                                        index, last_index - index);
                        page = find_get_page(mapping, index);
                }
                if (page && PageReadahead(page)) {
                        page_cache_readahead_adaptive(mapping,
                                        &ra, filp, page,
                                        index, last_index - index);
                }
        
Cache hot files neither have missing pages (!page) or lookahead
pages (PageReadahead(page)).  So it will not even be called.

> > > You seem to have a lot of magic numbers. They probably all need symbols and 
> > > explanations.
> > 
> > The magic numbers are for easier testings, and will be removed in
> > future.  For now, they enables convenient comparing of the two
> > algorithms in one kernel.
> 
> I mean the 16 and 4 not the sysctl

The numbers and the code in get_next_ra_size2() is simply copied from
get_next_ra_size():

        if (cur < max / 16) {
                newsize = 4 * cur;
        } else {
                newsize = 2 * cur;
        }

It's a trick to ramp up small sizes more quickly.
That trick is documented in the related get_init_ra_size().
So, it would be better to put the two routines together to make it clear.

> > 
> > If this new algorithm has been further tested and approved, I'll
> > re-submit the patch in a cleaner, standalone form. The adaptive
> > readahead patches can be dropped then. They may better be reworked as
> > a kernel module.
> 
> If they actually help and don't cause regressions they shouldn't be a module, 
> but integrated eventually Just it has to be all step by step.

Yeah, the adaptive readahead is complex and the possible workloads diverse.
It becomes obvious that there is a long way to go, and kernel module makes
life easier.

> > > Your white space also needs some work.
> > 
> > White space in patch description?
> 
> In the code indentation.

Ah, got it: a silly copy/paste mistake.

Thank you,
Wu

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