Re: [PATCH 05/12] vmscan: kill prev_priority completely

[Rik van Riel]

On 06/16/2010 07:37 PM, Andrew Morton wrote:

> This would have been badder in earlier days when we were using the
> scanning priority to decide when to start unmapping pte-mapped pages -
> page reclaim would have been recirculating large blobs of mapped pages
> around the LRU until the priority had built to the level where we
> started to unmap them.
>
> However that priority-based decision got removed and right now I don't
> recall what it got replaced with.  Aren't we now unmapping pages way
> too early and suffering an increased major&minor fault rate?  Worried.

We keep a different set of statistics to decide whether to
reclaim only page cache pages, or both page cache and
anonymous pages. The function get_scan_ratio parses those
statistics.

>    So.  What's up with that?  I don't even remember _why_ we disable
>    the swap token once the scanning priority gets severe and the code
>    comments there are risible.  And why do we wait until priority==0
>    rather than priority==1?

The reason is that we never page out the pages belonging to the
process owning the swap token (with the exception of that process
evicting its own pages).

If that process has a really large RSS in the current zone, and
we are having problems freeing pages, it may be beneficial to
also evict pages from that process.

Now that the LRU lists are split out into file backed and swap
backed, it may be a lot easier to find pages to evict.  That
may mean we could notice we're getting into trouble at much
higher priority levels and disable the swap token at a higher
priority level.

I do not believe prev_priority will be very useful here, since
we'd like to start out with small scans whenever possible.

> - Busted prev_priority means that lumpy reclaim will act oddly.
>    Every time someone goes into do some recalim, they'll start out not
>    doing lumpy reclaim.  Then, after a while, they'll get a clue and
>    will start doing the lumpy thing.  Then they return from reclaim and
>    the next recalim caller will again forget that he should have done
>    lumpy reclaim.

How common are lumpy reclaims, anyway?

Isn't it more likely that in-between every two higher-order
reclaims, a number of order zero reclaims will be happening?

In that case, the prev_priority logic may have introduced the
kind of behavioural bug you describe above...

> And one has to wonder: if we're making these incorrect decisions based
> upon a bogus view of the current scanning difficulty, why are these
> various priority-based thresholding heuristics even in there?  Are they
> doing anything useful?

The prev_priority code was useful when we had filesystem and
swap backed pages mixed on the same LRU list.

I am not convinced it still has any use.

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