Re: [PATCH -mm v2] mm: have order > 0 compaction start off where it left
From: Andrew Morton <akpm@linux-foundation.org>
Date: 2012-06-28 20:59:43
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On Thu, 28 Jun 2012 13:55:20 -0400 Rik van Riel [off-list ref] wrote:
Order > 0 compaction stops when enough free pages of the correct page order have been coalesced. When doing subsequent higher order allocations, it is possible for compaction to be invoked many times. However, the compaction code always starts out looking for things to compact at the start of the zone, and for free pages to compact things to at the end of the zone. This can cause quadratic behaviour, with isolate_freepages starting at the end of the zone each time, even though previous invocations of the compaction code already filled up all free memory on that end of the zone. This can cause isolate_freepages to take enormous amounts of CPU with certain workloads on larger memory systems. The obvious solution is to have isolate_freepages remember where it left off last time, and continue at that point the next time it gets invoked for an order > 0 compaction. This could cause compaction to fail if cc->free_pfn and cc->migrate_pfn are close together initially, in that case we restart from the end of the zone and try once more. Forced full (order == -1) compactions are left alone.
Is there a quality of service impact here? Newly-compactable pages at lower pfns than compact_cached_free_pfn will now get missed, leading to a form of fragmentation?
quoted hunk ↗ jump to hunk
@@ -463,6 +474,8 @@ static void isolate_freepages(struct zone *zone, */ if (isolated) high_pfn = max(high_pfn, pfn); + if (cc->order > 0) + zone->compact_cached_free_pfn = high_pfn;
Is high_pfn guaranteed to be aligned to pageblock_nr_pages here? I assume so, if lots of code in other places is correct but it's unobvious from reading this function.
quoted hunk ↗ jump to hunk
} /* split_free_page does not map the pages */ ...--- a/mm/internal.h +++ b/mm/internal.h@@ -118,8 +118,10 @@ struct compact_control { unsigned long nr_freepages; /* Number of isolated free pages */ unsigned long nr_migratepages; /* Number of pages to migrate */ unsigned long free_pfn; /* isolate_freepages search base */ + unsigned long start_free_pfn; /* where we started the search */ unsigned long migrate_pfn; /* isolate_migratepages search base */ bool sync; /* Synchronous migration */ + bool wrapped; /* Last round for order>0 compaction */
This comment is incomprehensible :(
int order; /* order a direct compactor needs */ int migratetype; /* MOVABLE, RECLAIMABLE etc */
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