Re: [PATCH 7/8] writeback: sync old inodes first in background writeback

[Wu Fengguang <fengguang.wu@intel.com>]

On Thu, Jul 22, 2010 at 06:48:23PM +0800, Mel Gorman wrote:
> On Thu, Jul 22, 2010 at 05:21:55PM +0800, Wu Fengguang wrote:
> > > I guess this new patch is more problem oriented and acceptable:
> > > 
> > > --- linux-next.orig/mm/vmscan.c	2010-07-22 16:36:58.000000000 +0800
> > > +++ linux-next/mm/vmscan.c	2010-07-22 16:39:57.000000000 +0800
> > > @@ -1217,7 +1217,8 @@ static unsigned long shrink_inactive_lis
> > >  			count_vm_events(PGDEACTIVATE, nr_active);
> > >  
> > >  			nr_freed += shrink_page_list(&page_list, sc,
> > > -							PAGEOUT_IO_SYNC);
> > > +					priority < DEF_PRIORITY / 3 ?
> > > +					PAGEOUT_IO_SYNC : PAGEOUT_IO_ASYNC);
> > >  		}
> > >  
> > >  		nr_reclaimed += nr_freed;
> > 
> > This one looks better:
> > ---
> > vmscan: raise the bar to PAGEOUT_IO_SYNC stalls
> > 
> > Fix "system goes totally unresponsive with many dirty/writeback pages"
> > problem:
> > 
> > 	http://lkml.org/lkml/2010/4/4/86
> > 
> > The root cause is, wait_on_page_writeback() is called too early in the
> > direct reclaim path, which blocks many random/unrelated processes when
> > some slow (USB stick) writeback is on the way.
> > 
> 
> So, what's the bet if lumpy reclaim is a factor that it's
> high-order-but-low-cost such as fork() that are getting caught by this since
> [78dc583d: vmscan: low order lumpy reclaim also should use PAGEOUT_IO_SYNC]
> was introduced?

Sorry I'm a bit confused by your wording..

> That could manifest to the user as stalls creating new processes when under
> heavy IO. I would be surprised it would freeze the entire system but certainly
> any new work would feel very slow.
> 
> > A simple dd can easily create a big range of dirty pages in the LRU
> > list. Therefore priority can easily go below (DEF_PRIORITY - 2) in a
> > typical desktop, which triggers the lumpy reclaim mode and hence
> > wait_on_page_writeback().
> > 
> 
> which triggers the lumpy reclaim mode for high-order allocations.

Exactly. Changelog updated.

> lumpy reclaim mode is not something that is triggered just because priority
> is high.

Right.

> I think there is a second possibility for causing stalls as well that is
> unrelated to lumpy reclaim. Once dirty_limit is reached, new page faults may
> also result in stalls. If it is taking a long time to writeback dirty data,
> random processes could be getting stalled just because they happened to dirty
> data at the wrong time.  This would be the case if the main dirtying process
> (e.g. dd) is not calling sync and dropping pages it's no longer using.

The dirty_limit throttling will slow down the dirty process to the
writeback throughput. If a process is dirtying files on sda (HDD),
it will be throttled at 80MB/s. If another process is dirtying files
on sdb (USB 1.1), it will be throttled at 1MB/s.

So dirty throttling will slow things down. However the slow down
should be smooth (a series of 100ms stalls instead of a sudden 10s
stall), and won't impact random processes (which does no read/write IO
at all).

> > In Andreas' case, 512MB/1024 = 512KB, this is way too low comparing to
> > the 22MB writeback and 190MB dirty pages. There can easily be a
> > continuous range of 512KB dirty/writeback pages in the LRU, which will
> > trigger the wait logic.
> > 
> > To make it worse, when there are 50MB writeback pages and USB 1.1 is
> > writing them in 1MB/s, wait_on_page_writeback() may stuck for up to 50
> > seconds.
> > 
> > So only enter sync write&wait when priority goes below DEF_PRIORITY/3,
> > or 6.25% LRU. As the default dirty throttle ratio is 20%, sync write&wait
> > will hardly be triggered by pure dirty pages.
> > 
> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > ---
> >  mm/vmscan.c |    4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > --- linux-next.orig/mm/vmscan.c	2010-07-22 16:36:58.000000000 +0800
> > +++ linux-next/mm/vmscan.c	2010-07-22 17:03:47.000000000 +0800
> > @@ -1206,7 +1206,7 @@ static unsigned long shrink_inactive_lis
> >  		 * but that should be acceptable to the caller
> >  		 */
> >  		if (nr_freed < nr_taken && !current_is_kswapd() &&
> > -		    sc->lumpy_reclaim_mode) {
> > +		    sc->lumpy_reclaim_mode && priority < DEF_PRIORITY / 3) {
> >  			congestion_wait(BLK_RW_ASYNC, HZ/10);
> >  
> 
> This will also delay waiting on congestion for really high-order
> allocations such as huge pages, some video decoder and the like which
> really should be stalling.

I absolutely agree that high order allocators should be somehow throttled.

However given that one can easily create a large _continuous_ range of
dirty LRU pages, let someone bumping all the way through the range
sounds a bit cruel..

> How about the following compile-tested diff?
> It takes the cost of the high-order allocation into account and the
> priority when deciding whether to synchronously wait or not.

Very nice patch. Thanks!

Cheers,
Fengguang

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 9c7e57c..d652e0c 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1110,6 +1110,48 @@ static int too_many_isolated(struct zone *zone, int file,
>  }
>  
>  /*
> + * Returns true if the caller should stall on congestion and retry to clean
> + * the list of pages synchronously.
> + *
> + * If we are direct reclaiming for contiguous pages and we do not reclaim
> + * everything in the list, try again and wait for IO to complete. This
> + * will stall high-order allocations but that should be acceptable to
> + * the caller
> + */
> +static inline bool should_reclaim_stall(unsigned long nr_taken,
> +				unsigned long nr_freed,
> +				int priority,
> +				struct scan_control *sc)
> +{
> +	int lumpy_stall_priority;
> +
> +	/* kswapd should not stall on sync IO */
> +	if (current_is_kswapd())
> +		return false;
> +
> +	/* Only stall on lumpy reclaim */
> +	if (!sc->lumpy_reclaim_mode)
> +		return false;
> +
> +	/* If we have relaimed everything on the isolated list, no stall */
> +	if (nr_freed == nr_taken)
> +		return false;
> +
> +	/*
> +	 * For high-order allocations, there are two stall thresholds.
> +	 * High-cost allocations stall immediately where as lower
> +	 * order allocations such as stacks require the scanning
> +	 * priority to be much higher before stalling
> +	 */
> +	if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
> +		lumpy_stall_priority = DEF_PRIORITY;
> +	else
> +		lumpy_stall_priority = DEF_PRIORITY / 3;
> +
> +	return priority <= lumpy_stall_priority;
> +}
> +
> +/*
>   * shrink_inactive_list() is a helper for shrink_zone().  It returns the number
>   * of reclaimed pages
>   */
> @@ -1199,14 +1241,8 @@ static unsigned long shrink_inactive_list(unsigned long max_scan,
>  		nr_scanned += nr_scan;
>  		nr_freed = shrink_page_list(&page_list, sc, PAGEOUT_IO_ASYNC);
>  
> -		/*
> -		 * If we are direct reclaiming for contiguous pages and we do
> -		 * not reclaim everything in the list, try again and wait
> -		 * for IO to complete. This will stall high-order allocations
> -		 * but that should be acceptable to the caller
> -		 */
> -		if (nr_freed < nr_taken && !current_is_kswapd() &&
> -		    sc->lumpy_reclaim_mode) {
> +		/* Check if we should syncronously wait for writeback */
> +		if (should_reclaim_stall(nr_taken, nr_freed, priority, sc)) {
>  			congestion_wait(BLK_RW_ASYNC, HZ/10);
>  
>  			/*
> 
> 

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