On Mon, Jul 26, 2010 at 08:57:17PM +0800, Mel Gorman wrote: > On Mon, Jul 26, 2010 at 07:27:09PM +0800, Wu Fengguang wrote: > > > > > @@ -933,13 +934,16 @@ keep_dirty: > > > > > VM_BUG_ON(PageLRU(page) || PageUnevictable(page)); > > > > > } > > > > > > > > > > + /* > > > > > + * If reclaim is encountering dirty pages, it may be because > > > > > + * dirty pages are reaching the end of the LRU even though > > > > > + * the dirty_ratio may be satisified. In this case, wake > > > > > + * flusher threads to pro-actively clean some pages > > > > > + */ > > > > > + wakeup_flusher_threads(laptop_mode ? 0 : nr_dirty + nr_dirty / 2); > > > > > > > > Ah it's very possible that nr_dirty==0 here! Then you are hitting the > > > > number of dirty pages down to 0 whether or not pageout() is called. > > > > > > > > > > True, this has been fixed to only wakeup flusher threads when this is > > > the file LRU, dirty pages have been encountered and the caller has > > > sc->may_writepage. > > > > OK. > > > > > > Another minor issue is, the passed (nr_dirty + nr_dirty / 2) is > > > > normally a small number, much smaller than MAX_WRITEBACK_PAGES. > > > > The flusher will sync at least MAX_WRITEBACK_PAGES pages, this is good > > > > for efficiency. > > > > And it seems good to let the flusher write much more > > > > than nr_dirty pages to safeguard a reasonable large > > > > vmscan-head-to-first-dirty-LRU-page margin. So it would be enough to > > > > update the comments. > > > > > > > > > > Ok, the reasoning had been to flush a number of pages that was related > > > to the scanning rate but if that is inefficient for the flusher, I'll > > > use MAX_WRITEBACK_PAGES. > > > > It would be better to pass something like (nr_dirty * N). > > MAX_WRITEBACK_PAGES may be increased to 128MB in the future, which is > > obviously too large as a parameter. When the batch size is increased > > to 128MB, the writeback code may be improved somehow to not exceed the > > nr_pages limit too much. > > > > What might be a useful value for N? 1.5 appears to work reasonably well > to create a window of writeback ahead of the scanner but it's a bit > arbitrary. I'd recommend N to be a large value. It's no longer relevant now since we'll call the flusher to sync some range containing the target page. The flusher will then choose an N large enough (eg. 4MB) for efficient IO. It needs to be a large value, otherwise the vmscan code will quickly run into dirty pages again.. 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>