On Wed, Jul 28, 2010 at 05:10:33PM +0800, Mel Gorman wrote: > On Wed, Jul 28, 2010 at 04:46:54PM +0800, Wu Fengguang wrote: > > The wait_on_page_writeback() call inside pageout() is virtually dead code. > > > > shrink_inactive_list() > > shrink_page_list(PAGEOUT_IO_ASYNC) > > pageout(PAGEOUT_IO_ASYNC) > > shrink_page_list(PAGEOUT_IO_SYNC) > > pageout(PAGEOUT_IO_SYNC) > > > > Because shrink_page_list/pageout(PAGEOUT_IO_SYNC) is always called after > > a preceding shrink_page_list/pageout(PAGEOUT_IO_ASYNC), the first > > pageout(ASYNC) converts dirty pages into writeback pages, the second > > shrink_page_list(SYNC) waits on the clean of writeback pages before > > calling pageout(SYNC). The second shrink_page_list(SYNC) can hardly run > > into dirty pages for pageout(SYNC) unless in some race conditions. > > > > It's possible for the second call to run into dirty pages as there is a > congestion_wait() call between the first shrink_page_list() call and the > second. That's a big window. OK there is a <=0.1s time window. Then what about the data set size? After first shrink_page_list(ASYNC), there will be hardly any pages left in the page_list except for the already under-writeback pages and other unreclaimable pages. So it still asks for some race conditions for hitting the second pageout(SYNC) -- some unreclaimable pages become reclaimable+dirty in the 0.1s time window. > > And the wait page-by-page behavior of pageout(SYNC) will lead to very > > long stall time if running into some range of dirty pages. > > True, but this is also lumpy reclaim which is depending on a contiguous > range of pages. It's better for it to wait on the selected range of pages > which is known to contain at least one old page than excessively scan and > reclaim newer pages. > > > So it's bad > > idea anyway to call wait_on_page_writeback() inside pageout(). > > > > I recognise that you are probably thinking of the stall-due-to-fork problem > but I'd expect the patch that raises the bar for <= PAGE_ALLOC_COSTLY_ORDER > to be sufficient. If not, I think it still makes sense to call > wait_on_page_writeback() for > PAGE_ALLOC_COSTLY_ORDER. The main intention of this patch is to remove semi-dead code. I'm less disturbed by the long stall time now with the previous patch ;) Thanks, Fengguang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>