tree 9b16bb9730979b319efe436318d0cc1d563c5676
parent cd9bb9b1192bf796c87e7ec9e9fc2d2c5b82e3ea
author Mel Gorman <mgorman@suse.de> 1350644410 +0100
committer Laura Abbott <lauraa@codeaurora.org> 1371576290 -0700

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.

Change-Id: I9db1a609fc86a95e3fd8d3774de994197ecb9adf
Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Git-commit: 397487db696cae0b026a474a5cd66f4e372995e6
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>