> On Wed, Jul 28, 2010 at 06:43:41PM +0900, KOSAKI Motohiro 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. > > > > > > > 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. > > > > Today, I was successful to reproduce the Andres's issue. and I disagree this > > opinion. > > Is Andres's issue not covered by the patch "vmscan: raise the bar to > PAGEOUT_IO_SYNC stalls" because wait_on_page_writeback() was the > main problem? Well, "vmscan: raise the bar to PAGEOUT_IO_SYNC stalls" is completely bandaid and much IO under slow USB flash memory device still cause such problem even if the patch is applied. But removing wait_on_page_writeback() doesn't solve the issue perfectly because current lumpy reclaim have multiple sick. again, I'm writing explaining mail..... -- 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>