Re: [PATCH 03/10] mm: vmscan: Flatten kswapd priority loop

From: Simon Jeons
Date: Tue Mar 19 2013 - 06:26:56 EST

Hi Mel,
On 03/19/2013 06:14 PM, Mel Gorman wrote:
On Tue, Mar 19, 2013 at 11:08:23AM +0800, Simon Jeons wrote:
Hi Mel,
On 03/17/2013 09:04 PM, Mel Gorman wrote:
kswapd stops raising the scanning priority when at least SWAP_CLUSTER_MAX
pages have been reclaimed or the pgdat is considered balanced. It then
rechecks if it needs to restart at DEF_PRIORITY and whether high-order
reclaim needs to be reset. This is not wrong per-se but it is confusing
per-se is short for what?

It means "in self" or "as such".

to follow and forcing kswapd to stay at DEF_PRIORITY may require several
restarts before it has scanned enough pages to meet the high watermark even
at 100% efficiency. This patch irons out the logic a bit by controlling
when priority is raised and removing the "goto loop_again".

This patch has kswapd raise the scanning priority until it is scanningmm: vmscan: Flatten kswapd priority loop
enough pages that it could meet the high watermark in one shrink of the
LRU lists if it is able to reclaim at 100% efficiency. It will not raise
Which kind of reclaim can be treated as 100% efficiency?

100% efficiency is where every page scanned can be reclaimed immediately.

- * We do this so kswapd doesn't build up large priorities for
- * example when it is freeing in parallel with allocators. It
- * matches the direct reclaim path behaviour in terms of impact
- * on zone->*_priority.
+ * Fragmentation may mean that the system cannot be rebalanced
+ * for high-order allocations in all zones. If twice the
+ * allocation size has been reclaimed and the zones are still
+ * not balanced then recheck the watermarks at order-0 to
+ * prevent kswapd reclaiming excessively. Assume that a
+ * process requested a high-order can direct reclaim/compact.
- if (sc.nr_reclaimed >= SWAP_CLUSTER_MAX)
- break;
- } while (--sc.priority >= 0);
+ if (order && sc.nr_reclaimed >= 2UL << order)
+ order = sc.order = 0;
If order == 0 is meet, should we do defrag for it?

Compaction is unnecessary for order-0.

I mean since order && sc.reclaimed >= 2UL << order, it is reclaimed for high order allocation, if order == 0 is meet, should we do defrag for it?

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