Re: madvise(2) MADV_SEQUENTIAL behavior

From: Eric Rannaud
Date: Wed Jul 16 2008 - 20:02:17 EST

On Wed, 2008-07-16 at 17:05 -0400, Chris Snook wrote:
> Rik van Riel wrote:
> > I believe that for mmap MADV_SEQUENTIAL, we will have to do
> > an unmap-behind from the fault path. Not every time, but
> > maybe once per megabyte, unmapping the megabyte behind us.
> Wouldn't it just be easier to not move pages to the active list when
> they're referenced via an MADV_SEQUENTIAL mapping? If we keep them on
> the inactive list, they'll be candidates for reclaiming, but they'll
> still be in pagecache when another task scans through, as long as we're
> not under memory pressure.

This approach, instead of invalidating the pages right away would
provide a middle ground: a way to tell the kernel "these pages are not
too important".

Whereas if MADV_SEQUENTIAL just invalidates the pages once per megabyte
(say), then it's only doing what is already possible using MADV_DONTNEED
("drop this pages now"). It would automate the process, but it would not
provide a more subtle hint, which could be quite useful.

As I see it, there are two basic concepts here:
- no_reuse (like FADV_NOREUSE)
- more_ra (more readahead)
(DONTNEED being another different concept)

MADV_SEQUENTIAL = more_ra | no_reuse
FADV_SEQUENTIAL = more_ra | no_reuse
FADV_NOREUSE = no_reuse

Right now, only the 'more_ra' part is implemented. 'no_reuse' could be
implemented as Chris suggests.

It looks like the disagreement a year ago around Peter's approach was
mostly around the question of whether using read ahead as a heuristic
for "drop behind" was safe for all workloads.

Would it be less controversial to remove the heuristic (ra->size ==
ra->ra_pages), and to do something only if the user asked for

It might encourage user space applications to start using
FADV_SEQUENTIAL or FADV_NOREUSE more often (as it would become
worthwhile to do so), and if they do (especially cron jobs), the problem
of the slow desktop in the morning would progressively solve itself.


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