Messages in this thread | | | Date | Thu, 15 Dec 2011 23:47:37 -0500 | From | Rik van Riel <> | Subject | Re: [PATCH 11/11] mm: Isolate pages for immediate reclaim on their own LRU |
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On 12/14/2011 10:41 AM, Mel Gorman wrote: > It was observed that scan rates from direct reclaim during tests > writing to both fast and slow storage were extraordinarily high. The > problem was that while pages were being marked for immediate reclaim > when writeback completed, the same pages were being encountered over > and over again during LRU scanning. > > This patch isolates file-backed pages that are to be reclaimed when > clean on their own LRU list.
The idea makes total sense to me. This is very similar to the inactive_laundry list in the early 2.4 kernel.
One potential issue is that the page cannot be moved back to the active list by mark_page_accessed(), which would have to be taught about the immediate LRU.
> @@ -255,24 +256,80 @@ static void pagevec_move_tail(struct pagevec *pvec) > } > > /* > + * Similar pair of functions to pagevec_move_tail except it is called when > + * moving a page from the LRU_IMMEDIATE to one of the [in]active_[file|anon] > + * lists > + */ > +static void pagevec_putback_immediate_fn(struct page *page, void *arg) > +{ > + struct zone *zone = page_zone(page); > + > + if (PageLRU(page)) { > + enum lru_list lru = page_lru(page); > + list_move(&page->lru,&zone->lru[lru].list); > + } > +}
Should this not put the page at the reclaim end of the inactive list, since we want to try evicting it?
> + /* > + * There is a potential race that if a page is set PageReclaim > + * and moved to the LRU_IMMEDIATE list after writeback completed, > + * it can be left on the LRU_IMMEDATE list with no way for > + * reclaim to find it. > + * > + * This race should be very rare but count how often it happens. > + * If it is a continual race, then it's very unsatisfactory as there > + * is no guarantee that rotate_reclaimable_page() will be called > + * to rescue these pages but finding them in page reclaim is also > + * problematic due to the problem of deciding when the right time > + * to scan this list is. > + */
Would it be an idea for the pageout code to check whether the page at the head of the LRU_IMMEDIATE list is freeable, and then take that page?
Of course, that does mean adding a check to rotate_reclaimable_page to make sure the page is still on the LRU_IMMEDIATE list, and did not get moved by somebody else...
Also, it looks like your debugging check can trigger even when the bug does not happen (on the last LRU_IMMEDIATE page), because you decrement NR_IMMEDIATE before you get to this check.
-- All rights reversed
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