Re: Drop PageReclaim()

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 8 Feb 2007 17:06:33 -0800 (PST)
Christoph Lameter <clameter@sgi.com> wrote:

> On Thu, 8 Feb 2007, Andrew Morton wrote:
> 
> > I doubt it.  One would need to troll five-year-old changelogs and mailing
> > list discussion, but iirc that rotation was a large win in certain
> > workloads, preventing scanning meltdowns and general memory stress.
> 
> I'd expect trouble here because of the taking of a LRU lock per page.

PG_reclaim is there to prevent that problem (amongst other things).

If the proportion of written-back pages due to the page scanner is large,
things already suck.  The VM tries to minimise that IO and to maximise the
inode-based writeback.

> For 
> large amounts of concurrent I/O this could be an issue.
> 
> > > One additional issue that is raised by the writeback pages remaining on 
> > > the LRU lists is that we can get into the same livelock situation as with 
> > > mlocked pages if we keep on skipping over writeback pages.
> > 
> > That's why we rotate the reclaimable pages back to the head-of-queue.
> 
> I think the reclaim writeout is one minor contributor here.

To what?

> If there are 
> large amounts of writeback pages from f.e. streaming general I/O then we 
> may run still into bad situations because we need to scan over them.

If the inactive list is small relative to the number of under-writeback
pages in the zone then there could be problems there.  But we just throttle
and wait for some pages to come clean, which seems to work OK.


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