Re: [PATCH 5/5] linear reclaim core

Andrew Morton
2006-09-08T18:41:14Z

On Fri, 8 Sep 2006 13:27:18 +0100
Andy Whitcroft <apw@shadowen.org> wrote:

> When we are out of memory of a suitable size we enter reclaim.
> The current reclaim algorithm targets pages in LRU order, which
> is great for fairness but highly unsuitable if you desire pages at
> higher orders. To get pages of higher order we must shoot down a
> very high proportion of memory; >95% in a lot of cases.
>
> This patch introduces an alternative algorithm used when requesting
> higher order allocations. Here we look at memory in ranges at the
> order requested. We make a quick pass to see if all pages in that
> area are likely to be reclaimed, only then do we apply reclaim to
> the pages in the area.
>
> Testing in combination with fragmentation avoidance shows
> significantly improved chances of a successful allocation at
> higher order.

I bet it does.

I'm somewhat surprised at the implementation. Would it not be sufficient
to do this within shrink_inactive_list()? Something along the lines of:

- Pick tail page off LRU.

- For all "neighbour" pages (alignment == 1<<order, count == 1<<order)

- If they're all PageLRU and !PageActive, add them all to page_list for
possible reclaim

And, in shrink_active_list:

- Pick tail page off LRU

- For all "neighbour" pages (alignment == 1<<order, count == 1<<order)

If they're all PageLRU, put all the active pages in this block onto
l_hold for possible deactivation.

Maybe all that can be done in isolate_lru_pages().