Re: [PATCH] vmscan: limit concurrent reclaimers in shrink_zone

[Andi Kleen <andi@firstfloor.org>]

Rik van Riel <riel@redhat.com> writes:

> +max_zone_concurrent_reclaim:
> +
> +The number of processes that are allowed to simultaneously reclaim
> +memory from a particular memory zone.
> +
> +With certain workloads, hundreds of processes end up in the page
> +reclaim code simultaneously.  This can cause large slowdowns due
> +to lock contention, freeing of way too much memory and occasionally
> +false OOM kills.
> +
> +To avoid these problems, only allow a smaller number of processes
> +to reclaim pages from each memory zone simultaneously.
> +
> +The default value is 8.

I don't like the hardcoded number. Is the same number good for a 128MB
embedded system as for as 1TB server?  Seems doubtful.

This should be perhaps scaled with memory size and number of CPUs?

> +/*
> + * Maximum number of processes concurrently running the page
> + * reclaim code in a memory zone.  Having too many processes
> + * just results in them burning CPU time waiting for locks,
> + * so we're better off limiting page reclaim to a sane number
> + * of processes at a time.  We do this per zone so local node
> + * reclaim on one NUMA node will not block other nodes from
> + * making progress.
> + */
> +int max_zone_concurrent_reclaimers = 8;

__read_mostly

> +
>  static LIST_HEAD(shrinker_list);
>  static DECLARE_RWSEM(shrinker_rwsem);
>  
> @@ -1600,6 +1612,29 @@ static void shrink_zone(int priority, struct zone *zone,
>  	struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
>  	int noswap = 0;
>  
> +	if (!current_is_kswapd() && atomic_read(&zone->concurrent_reclaimers) >
> +					max_zone_concurrent_reclaimers) {
> +		/*
> +		 * Do not add to the lock contention if this zone has
> +		 * enough processes doing page reclaim already, since
> +		 * we would just make things slower.
> +		 */
> +		sleep_on(&zone->reclaim_wait);

wait_event()? sleep_on is a really deprecated racy interface.

This would still badly thunder the herd if not enough memory is freed
, won't it? It would be better to only wake up a single process if memory got freed.

How about for each page freed do a wake up for one thread?


-Andi
-- 
ak@linux.intel.com -- Speaking for myself only.