Re: [PATCH 02/10] mm: vmscan: Obey proportional scanning requirements for kswapd

[Mel Gorman <mgorman@suse.de>]

On Fri, Mar 22, 2013 at 03:09:02PM -0400, Johannes Weiner wrote:
> > To preserve existing behaviour, that makes sense. I'm not convinced that
> > it's necessarily the best idea but altering it would be beyond the scope
> > of this series and bite off more than I'm willing to chew. This actually
> > simplifies things a bit and shrink_lruvec turns into the (untested) code
> > below. It does not do exact proportional scanning but I do not think it's
> > necessary to either and is a useful enough approximation. It still could
> > end up reclaiming much more than sc->nr_to_reclaim unfortunately but fixing
> > it requires reworking how kswapd scans at different priorities.
> 
> In which way does it not do exact proportional scanning?  I commented
> on one issue below, but maybe you were referring to something else.
> 

You guessed what I was referring to correctly.

> Yes, it's a little unfortunate that we escalate to a gigantic scan
> window first, and then have to contort ourselves in the process of
> backing off gracefully after we reclaimed a few pages...
> 

The next patch "mm: vmscan: Flatten kswapd priority loop" mitigates the
problem slightly by improving how kswapd controls when priority gets raised.
It's not perfect though, lots of pages under writeback at the tail of
the LRU will still raise the priority quickly.

> > Is this closer to what you had in mind?
> > 
> > static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> > {
> > 	unsigned long nr[NR_LRU_LISTS];
> > 	unsigned long nr_to_scan;
> > 	enum lru_list lru;
> > 	unsigned long nr_reclaimed = 0;
> > 	unsigned long nr_to_reclaim = sc->nr_to_reclaim;
> > 	unsigned long nr_anon_scantarget, nr_file_scantarget;
> > 	struct blk_plug plug;
> > 	bool scan_adjusted = false;
> > 
> > 	get_scan_count(lruvec, sc, nr);
> > 
> > 	/* Record the original scan target for proportional adjustments later */
> > 	nr_file_scantarget = nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE] + 1;
> > 	nr_anon_scantarget = nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON] + 1;
> > 
> > 	blk_start_plug(&plug);
> > 	while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
> > 					nr[LRU_INACTIVE_FILE]) {
> > 		unsigned long nr_anon, nr_file, percentage;
> > 
> > 		for_each_evictable_lru(lru) {
> > 			if (nr[lru]) {
> > 				nr_to_scan = min(nr[lru], SWAP_CLUSTER_MAX);
> > 				nr[lru] -= nr_to_scan;
> > 
> > 				nr_reclaimed += shrink_list(lru, nr_to_scan,
> > 							    lruvec, sc);
> > 			}
> > 		}
> > 
> > 		if (nr_reclaimed < nr_to_reclaim || scan_adjusted)
> > 			continue;
> > 
> > 		/*
> > 		 * For global direct reclaim, reclaim only the number of pages
> > 		 * requested. Less care is taken to scan proportionally as it
> > 		 * is more important to minimise direct reclaim stall latency
> > 		 * than it is to properly age the LRU lists.
> > 		 */
> > 		if (global_reclaim(sc) && !current_is_kswapd())
> > 			break;
> > 
> > 		/*
> > 		 * For kswapd and memcg, reclaim at least the number of pages
> > 		 * requested. Ensure that the anon and file LRUs shrink
> > 		 * proportionally what was requested by get_scan_count(). We
> > 		 * stop reclaiming one LRU and reduce the amount scanning
> > 		 * proportional to the original scan target.
> > 		 */
> > 		nr_file = nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE];
> > 		nr_anon = nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON];
> > 
> > 		if (nr_file > nr_anon) {
> > 			lru = LRU_BASE;
> > 			percentage = nr_anon * 100 / nr_anon_scantarget;
> > 		} else {
> > 			lru = LRU_FILE;
> > 			percentage = nr_file * 100 / nr_file_scantarget;
> > 		}
> > 
> > 		/* Stop scanning the smaller of the LRU */
> > 		nr[lru] = 0;
> > 		nr[lru + LRU_ACTIVE] = 0;
> > 
> > 		/* Reduce scanning of the other LRU proportionally */
> > 		lru = (lru == LRU_FILE) ? LRU_BASE : LRU_FILE;
> > 		nr[lru] = nr[lru] * percentage / 100;;
> > 		nr[lru + LRU_ACTIVE] = nr[lru + LRU_ACTIVE] * percentage / 100;
> 
> The percentage is taken from the original goal but then applied to the
> remainder of scan goal for the LRUs we continue scanning.  The more
> pages that have already been scanned, the more inaccurate this gets.
> Is that what you had in mind with useful enough approximation?

Yes. I could record the original scan rates, recalculate as a percentage
and then do something like

nr[lru] = min(nr[lru], origin_nr[lru] * percentage / 100)

but it was not obvious that the result would be any better.


-- 
Mel Gorman
SUSE Labs

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