Re: [PATCH 1/2] mm: page allocator: Initialise ZLC for first zoneeligible for zone_reclaim

From: Christoph Lameter
Date: Mon Jul 18 2011 - 13:20:40 EST

On Mon, 18 Jul 2011, Mel Gorman wrote:

> On Mon, Jul 18, 2011 at 09:56:31AM -0500, Christoph Lameter wrote:
> > On Fri, 15 Jul 2011, Mel Gorman wrote:
> >
> > > Currently the zonelist cache is setup only after the first zone has
> > > been considered and zone_reclaim() has been called. The objective was
> > > to avoid a costly setup but zone_reclaim is itself quite expensive. If
> > > it is failing regularly such as the first eligible zone having mostly
> > > mapped pages, the cost in scanning and allocation stalls is far higher
> > > than the ZLC initialisation step.
> >
> > Would it not be easier to set zlc_active and allowednodes based on the
> > zone having an active ZLC at the start of get_pages()?
> >
> What do you mean by a zones active ZLC? zonelists are on a per-node,
> not a per-zone basis (see node_zonelist) so a zone doesn't have an
> active ZLC as such. If the zlc_active is set at the beginning of

Look at get_page_from_freelist(): It sets
zlc_active = 0 even through the zonelist under consideration may have a
ZLC. zlc_active = 0 can also mean that the function has not bothered to
look for the zlc information of the current zonelist.

> get_page_from_freelist(), it implies that we are calling zlc_setup()
> even when the watermarks are met which is unnecessary.

Ok then that decision to not call zlc_setup() for performance reasons is
what created the problem that you are trying to solve. In case that the
first zones watermarks are okay we can avoid calling zlc_setup().

What we do now have is checking for zlc_active in the loop just so that
the first time around we do not call zlc_setup().

We may be able to simplify the function by:

1. Checking for the special case that the first zone is ok and that we do
not want to call zlc_setup before we get to the loop.

2. Do the zlc_setup() before the loop.

3. Remove the zlc_setup() code as you did from the loop as well as the
checks for zlc_active. zlc_active becomes not necessary since a zlc
is always available when we go through the loop.

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