On 07/09/15 at 06:13P, Alan Cox wrote: > > On Jul 8, 2015, at 1:29 PM, hiren panchasara <hi...@freebsd.org> wrote: > > > On 07/08/15 at 05:46P, Alan Cox wrote: > >> Author: alc > >> Date: Wed Jul 8 17:45:59 2015 > >> New Revision: 285282 > >> URL: https://svnweb.freebsd.org/changeset/base/285282 > >> > >> Log: > >> The intention of r254304 was to scan the active queue continuously. > >> However, I've observed the active queue scan stopping when there are > >> frequent free page shortages and the inactive queue is steadily refilled > >> by other mechanisms, such as the sequential access heuristic in vm_fault() > >> or madvise(2). > > > > What would be the actual downside/effect of this scenario? i.e. What goes > > wrong when active queue scan stops and what would I see/observe on a > > system when the problem is going on - is what I want to understand. > > > > > Basically, pages that haven?t been used recently get stuck in the active > queue. Consequently, they are never reclaimed and repurposed to hold newly > accessed data. Instead, more recently used pages from the inactive queue are > reclaimed. Effectively, this reduces the amount of physical memory that is > available for caching file data, and so the system (likely) performs more I/O > operations than it would have with the stuck pages problem corrected. >
Thanks. So it seems now there is an enforced number of min pages that
*must* be scanned which is proportional to the time of last scan.
Cheers,
Hiren
>
> >> To remedy this problem, record the time of the last active
> >> queue scan, and always scan a number of pages proportional to the time
> >> since the last scan, regardless of whether that last scan was a
> >> timeout-triggered ("pass == 0") or free-page-shortage-triggered ("pass >
> >> 0") scan.
> >>
> >> Also, on a timeout-triggered scan, allow a full scan of the active queue
> >> when the system is short of inactive pages.
> >>
> >> Reviewed by: kib
> >> MFC after: 6 weeks
> >> Sponsored by: EMC / Isilon Storage Division
> >
> > [skip]
> >
> > Thanks in advance.
> > Cheers,
> > Hiren
>
>
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