On Thu, Sep 16, 2010 at 05:13:38PM +0900, Minchan Kim wrote:
> On Wed, Sep 15, 2010 at 01:27:51PM +0100, Mel Gorman wrote:
> > If wait_iff_congested() is called with no BDI congested, the function simply
> > calls cond_resched(). In the event there is significant writeback happening
> > in the zone that is being reclaimed, this can be a poor decision as reclaim
> > would succeed once writeback was completed. Without any backoff logic,
> > younger clean pages can be reclaimed resulting in more reclaim overall and
> > poor performance.
>
> I agree.
>
> >
> > This patch tracks how many pages backed by a congested BDI were found during
> > scanning. If all the dirty pages encountered on a list isolated from the
> > LRU belong to a congested BDI, the zone is marked congested until the zone
>
> I am not sure it works well.
Check the competion times for the micro-mapped-file-stream benchmark in
the leader mail. Backing off like this is faster overall for some
workloads.
> We just met the condition once but we backoff it until high watermark.
Reaching the high watermark is considered to be a relieving of pressure.
> (ex, 32 isolated dirty pages == 32 pages on congestioned bdi)
> First impression is rather _aggressive_.
>
Yes, it is. I intended to start with something quite aggressive that is
close to existing behaviour and then experiment with alternatives.
For example, I considered clearing zone congestion when but nr_bdi_congested
drops to 0. This would be less aggressive in terms of congestion waiting but
it is further from todays behaviour. I felt it would be best to introduce
wait_iff_congested() in one kernel cycle but wait to a later cycle to deviate
a lot from congestion_wait().
> How about more checking?
> For example, if above pattern continues repeately above some threshold,
> we can regard "zone is congested" and then if the pattern isn't repeated
> during some threshold, we can regard "zone isn't congested any more.".
>
I also considered these options and got stuck at what the "some
threshold" is and how to record the history. Should it be recorded on a
per BDI basis for example? I think all these questions can be answered
but should be in a different cycle.
> > reaches the high watermark. wait_iff_congested() then checks both the
> > number of congested BDIs and if the current zone is one that has encounted
> > congestion recently, it will sleep on the congestion queue. Otherwise it
> > will call cond_reched() to yield the processor if necessary.
> >
> > The end result is that waiting on the congestion queue is avoided when
> > necessary but when significant congestion is being encountered,
> > reclaimers and page allocators will back off.
> >
> > Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> > ---
> > include/linux/backing-dev.h | 2 +-
> > include/linux/mmzone.h | 8 ++++
> > mm/backing-dev.c | 23 ++++++++----
> > mm/page_alloc.c | 4 +-
> > mm/vmscan.c | 83 +++++++++++++++++++++++++++++++++++++------
> > 5 files changed, 98 insertions(+), 22 deletions(-)
> >
> > diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> > index 72bb510..f1b402a 100644
> > --- a/include/linux/backing-dev.h
> > +++ b/include/linux/backing-dev.h
> > +static enum bdi_queue_status may_write_to_queue(struct backing_dev_info *bdi,
>
> <snip>
>
> > struct scan_control *sc)
> > {
> > + enum bdi_queue_status ret = QUEUEWRITE_DENIED;
> > +
> > if (current->flags & PF_SWAPWRITE)
> > - return 1;
> > + return QUEUEWRITE_ALLOWED;
> > if (!bdi_write_congested(bdi))
> > - return 1;
> > + return QUEUEWRITE_ALLOWED;
> > + else
> > + ret = QUEUEWRITE_CONGESTED;
> > if (bdi == current->backing_dev_info)
> > - return 1;
> > + return QUEUEWRITE_ALLOWED;
> >
> > /* lumpy reclaim for hugepage often need a lot of write */
> > if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
> > - return 1;
> > - return 0;
> > + return QUEUEWRITE_ALLOWED;
> > + return ret;
> > }
>
> The function can't return QUEUEXXX_DENIED.
> It can affect disable_lumpy_reclaim.
>
Yes, but that change was made in "vmscan: Narrow the scenarios lumpy
reclaim uses synchrounous reclaim". Maybe I am misunderstanding your
objection.
> >
> > /*
> > @@ -352,6 +362,8 @@ static void handle_write_error(struct address_space *mapping,
> > typedef enum {
> > /* failed to write page out, page is locked */
> > PAGE_KEEP,
> > + /* failed to write page out due to congestion, page is locked */
> > + PAGE_KEEP_CONGESTED,
> > /* move page to the active list, page is locked */
> > PAGE_ACTIVATE,
> > /* page has been sent to the disk successfully, page is unlocked */
> > @@ -401,9 +413,14 @@ static pageout_t pageout(struct page *page, struct address_space *mapping,
> > }
> > if (mapping->a_ops->writepage == NULL)
> > return PAGE_ACTIVATE;
> > - if (!may_write_to_queue(mapping->backing_dev_info, sc)) {
> > + switch (may_write_to_queue(mapping->backing_dev_info, sc)) {
> > + case QUEUEWRITE_CONGESTED:
> > + return PAGE_KEEP_CONGESTED;
> > + case QUEUEWRITE_DENIED:
> > disable_lumpy_reclaim_mode(sc);
> > return PAGE_KEEP;
> > + case QUEUEWRITE_ALLOWED:
> > + ;
> > }
> >
> > if (clear_page_dirty_for_io(page)) {
> > @@ -682,11 +699,14 @@ static noinline_for_stack void free_page_list(struct list_head *free_pages)
> > * shrink_page_list() returns the number of reclaimed pages
> > */
> > static unsigned long shrink_page_list(struct list_head *page_list,
> > + struct zone *zone,
> > struct scan_control *sc)
> > {
> > LIST_HEAD(ret_pages);
> > LIST_HEAD(free_pages);
> > int pgactivate = 0;
> > + unsigned long nr_dirty = 0;
> > + unsigned long nr_congested = 0;
> > unsigned long nr_reclaimed = 0;
> >
> > cond_resched();
> > @@ -706,6 +726,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > goto keep;
> >
> > VM_BUG_ON(PageActive(page));
> > + VM_BUG_ON(page_zone(page) != zone);
> >
> > sc->nr_scanned++;
> >
> > @@ -783,6 +804,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > }
> >
> > if (PageDirty(page)) {
> > + nr_dirty++;
> > +
> > if (references == PAGEREF_RECLAIM_CLEAN)
> > goto keep_locked;
> > if (!may_enter_fs)
> > @@ -792,6 +815,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> >
> > /* Page is dirty, try to write it out here */
> > switch (pageout(page, mapping, sc)) {
> > + case PAGE_KEEP_CONGESTED:
> > + nr_congested++;
> > case PAGE_KEEP:
> > goto keep_locked;
> > case PAGE_ACTIVATE:
> > @@ -903,6 +928,15 @@ keep_lumpy:
> > VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
> > }
> >
> > + /*
> > + * Tag a zone as congested if all the dirty pages encountered were
> > + * backed by a congested BDI. In this case, reclaimers should just
> > + * back off and wait for congestion to clear because further reclaim
> > + * will encounter the same problem
> > + */
> > + if (nr_dirty == nr_congested)
> > + zone_set_flag(zone, ZONE_CONGESTED);
> > +
> > free_page_list(&free_pages);
> >
> > list_splice(&ret_pages, page_list);
> > @@ -1387,12 +1421,12 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
> >
> > spin_unlock_irq(&zone->lru_lock);
> >
> > - nr_reclaimed = shrink_page_list(&page_list, sc);
> > + nr_reclaimed = shrink_page_list(&page_list, zone, sc);
> >
> > /* Check if we should syncronously wait for writeback */
> > if (should_reclaim_stall(nr_taken, nr_reclaimed, priority, sc)) {
> > set_lumpy_reclaim_mode(priority, sc, true);
> > - nr_reclaimed += shrink_page_list(&page_list, sc);
> > + nr_reclaimed += shrink_page_list(&page_list, zone, sc);
> > }
> >
> > local_irq_disable();
> > @@ -1940,8 +1974,26 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
> >
> > /* Take a nap, wait for some writeback to complete */
> > if (!sc->hibernation_mode && sc->nr_scanned &&
> > - priority < DEF_PRIORITY - 2)
> > - congestion_wait(BLK_RW_ASYNC, HZ/10);
> > + priority < DEF_PRIORITY - 2) {
> > + struct zone *active_zone = NULL;
> > + unsigned long max_writeback = 0;
> > + for_each_zone_zonelist(zone, z, zonelist,
> > + gfp_zone(sc->gfp_mask)) {
> > + unsigned long writeback;
> > +
> > + /* Initialise for first zone */
> > + if (active_zone == NULL)
> > + active_zone = zone;
> > +
> > + writeback = zone_page_state(zone, NR_WRITEBACK);
> > + if (writeback > max_writeback) {
> > + max_writeback = writeback;
> > + active_zone = zone;
> > + }
> > + }
> > +
> > + wait_iff_congested(active_zone, BLK_RW_ASYNC, HZ/10);
> > + }
>
> Other place just considers preferred zone.
> What is the rationale that consider max writeback zone in all zone of zonelist to
> call wait_iff_congeested?
Initially, it was because wait_iff_congested() heuristic was based on
writeback, not zone congestion. This time around, it was because I
wanted to be aggressive in terms of triggering the congestion wait to be
better than existing behaviour but not too far from it.
> Maybe max writeback zone can be much slow bdi but this process could be not related
> to the bdi. It can make random stall by point of view of this proces.
>
Fair point, I will retest using the preferred zone.
> > }
> >
> > out:
> > @@ -2251,6 +2303,15 @@ loop_again:
> > if (!zone_watermark_ok(zone, order,
> > min_wmark_pages(zone), end_zone, 0))
> > has_under_min_watermark_zone = 1;
> > + } else {
> > + /*
> > + * If a zone reaches its high watermark,
> > + * consider it to be no longer congested. It's
> > + * possible there are dirty pages backed by
> > + * congested BDIs but as pressure is relieved,
> > + * spectulatively avoid congestion waits
> > + */
> > + zone_clear_flag(zone, ZONE_CONGESTED);
> > }
> >
> > }
> > --
> > 1.7.1
> >
>
> --
> Kind regards,
> Minchan Kim
>
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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