[patch] mm, thp: do not perform sync compaction on pagefault

[patch] mm, thp: do not perform sync compaction on pagefault

[David Rientjes]

Synchronous memory compaction can be very expensive: it can iterate an enormous 
amount of memory without aborting and it can wait on page locks and writeback to 
complete if a pageblock cannot be defragmented.

Unfortunately, it's too expensive for pagefault for transparent hugepages and 
it's much better to simply fallback to pages.  On 128GB machines, we find that 
synchronous memory compaction can take O(seconds) for a single thp fault.

Signed-off-by: David Rientjes <rientjes@google.com>
---
 mm/page_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2656,7 +2656,7 @@ rebalance:
 		/* Wait for some write requests to complete then retry */
 		wait_iff_congested(preferred_zone, BLK_RW_ASYNC, HZ/50);
 		goto rebalance;
-	} else {
+	} else if (!(gfp_mask & __GFP_NO_KSWAPD)) {
 		/*
 		 * High-order allocations do not necessarily loop after
 		 * direct reclaim and reclaim/compaction depends on compaction

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Re: [patch] mm, thp: do not perform sync compaction on pagefault

[Joonsoo Kim]

2014-05-01 9:45 GMT+09:00 David Rientjes <rientjes@google.com>:
> Synchronous memory compaction can be very expensive: it can iterate an enormous
> amount of memory without aborting and it can wait on page locks and writeback to
> complete if a pageblock cannot be defragmented.
> Unfortunately, it's too expensive for pagefault for transparent hugepages and
> it's much better to simply fallback to pages.  On 128GB machines, we find that
> synchronous memory compaction can take O(seconds) for a single thp fault.

Hello,

AFAIK, synchronous compaction doesn't wait the page on writeback.
sync compaction pass MIGRATE_SYNC_LIGHT for migrate_mode,
instead of MIGRATE_SYNC. It results in skipping the page on writeback.

Thanks.

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