On Mon 23-01-17 13:16:40, Johannes Weiner wrote: > Dirty pages can easily reach the end of the LRU while there are still > clean pages to reclaim around. Don't let kswapd write them back just > because there are a lot of them. It costs more CPU to find the clean > pages, but that's almost certainly better than to disrupt writeback > from the flushers with LRU-order single-page writes from reclaim. And > the flushers have been woken up by that point, so we spend IO capacity > on flushing and CPU capacity on finding the clean cache. > > Only start writing dirty pages if they have cycled around the LRU > twice now and STILL haven't been queued on the IO device. It's > possible that the dirty pages are so sparsely distributed across > different bdis, inodes, memory cgroups, that the flushers take forever > to get to the ones we want reclaimed. Once we see them twice on the > LRU, we know that's the quicker way to find them, so do LRU writeback. > > Signed-off-by: Johannes Weiner <[email protected]>
Acked-by: Michal Hocko <[email protected]> > --- > mm/vmscan.c | 15 ++++++++++----- > 1 file changed, 10 insertions(+), 5 deletions(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 915fc658de41..df0fe0cc438e 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -1153,13 +1153,18 @@ static unsigned long shrink_page_list(struct > list_head *page_list, > > if (PageDirty(page)) { > /* > - * Only kswapd can writeback filesystem pages to > - * avoid risk of stack overflow but only writeback > - * if many dirty pages have been encountered. > + * Only kswapd can writeback filesystem pages > + * to avoid risk of stack overflow. But avoid > + * injecting inefficient single-page IO into > + * flusher writeback as much as possible: only > + * write pages when we've encountered many > + * dirty pages, and when we've already scanned > + * the rest of the LRU for clean pages and see > + * the same dirty pages again (PageReclaim). > */ > if (page_is_file_cache(page) && > - (!current_is_kswapd() || > - !test_bit(PGDAT_DIRTY, > &pgdat->flags))) { > + (!current_is_kswapd() || !PageReclaim(page) || > + !test_bit(PGDAT_DIRTY, &pgdat->flags))) { > /* > * Immediately reclaim when written back. > * Similar in principal to deactivate_page() > -- > 2.11.0 > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to [email protected]. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"[email protected]";> [email protected] </a> -- Michal Hocko SUSE Labs
