• M
    mm: compaction: Add scanned and isolated counters for compaction · 397487db
    Mel Gorman 提交于
    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: NMel Gorman <mgorman@suse.de>
    Reviewed-by: NRik van Riel <riel@redhat.com>
    397487db
vm_event_item.h 1.8 KB