Messages in this thread | | | Subject | Re: madvise(2) MADV_SEQUENTIAL behavior | From | Eric Rannaud <> | Date | Thu, 17 Jul 2008 00:01:50 +0000 |
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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)
Then: 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 _SEQUENTIAL or _NOREUSE?
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.
Thanks.
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