Commit 397487db authored Oct 19, 2012 by Mel Gorman

mm: compaction: Add scanned and isolated counters for compaction



Compaction already has tracepoints to count scanned and isolated pages
but it requires that ftrace be enabled and if that information has to be
written to disk then it can be disruptive. This patch adds vmstat counters
for compaction called compact_migrate_scanned, compact_free_scanned and
compact_isolated.

With these counters, it is possible to define a basic cost model for
compaction. This approximates of how much work compaction is doing and can
be compared that with an oprofile showing TLB misses and see if the cost of
compaction is being offset by THP for example. Minimally a compaction patch
can be evaluated in terms of whether it increases or decreases cost. The
basic cost model looks like this

Fundamental unit u:	a word	sizeof(void *)

Ca  = cost of struct page access = sizeof(struct page) / u

Cmc = Cost migrate page copy = (Ca + PAGE_SIZE/u) * 2
Cmf = Cost migrate failure   = Ca * 2
Ci  = Cost page isolation    = (Ca + Wi)
	where Wi is a constant that should reflect the approximate
	cost of the locking operation.

Csm = Cost migrate scanning = Ca
Csf = Cost free    scanning = Ca

Overall cost =	(Csm * compact_migrate_scanned) +
	      	(Csf * compact_free_scanned)    +
	      	(Ci  * compact_isolated)	+
		(Cmc * pgmigrate_success)	+
		(Cmf * pgmigrate_failed)

Where the values are read from /proc/vmstat.

This is very basic and ignores certain costs such as the allocation cost
to do a migrate page copy but any improvement to the model would still
use the same vmstat counters.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>

parent 7b2a2d4a

include/linux/vm_event_item.h

+2 −0

Original line number	Diff line number	Diff line
		@@ -42,6 +42,8 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
		PGMIGRATE_SUCCESS, PGMIGRATE_FAIL,
		#endif
		#ifdef CONFIG_COMPACTION
		COMPACTMIGRATE_SCANNED, COMPACTFREE_SCANNED,
		COMPACTISOLATED,
		COMPACTSTALL, COMPACTFAIL, COMPACTSUCCESS,
		#endif
		#ifdef CONFIG_HUGETLB_PAGE

mm/compaction.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -356,6 +356,10 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
		if (blockpfn == end_pfn)
		update_pageblock_skip(cc, valid_page, total_isolated, false);

		count_vm_events(COMPACTFREE_SCANNED, nr_scanned);
		if (total_isolated)
		count_vm_events(COMPACTISOLATED, total_isolated);

		return total_isolated;
		}

		@@ -646,6 +650,10 @@ next_pageblock:

		trace_mm_compaction_isolate_migratepages(nr_scanned, nr_isolated);

		count_vm_events(COMPACTMIGRATE_SCANNED, nr_scanned);
		if (nr_isolated)
		count_vm_events(COMPACTISOLATED, nr_isolated);

		return low_pfn;
		}

mm/vmstat.c

+3 −0

Original line number	Diff line number	Diff line
		@@ -779,6 +779,9 @@ const char * const vmstat_text[] = {
		"pgmigrate_fail",
		#endif
		#ifdef CONFIG_COMPACTION
		"compact_migrate_scanned",
		"compact_free_scanned",
		"compact_isolated",
		"compact_stall",
		"compact_fail",
		"compact_success",

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