Re: [RFC v2 PATCH] mm: vmscan: do not iterate all mem cgroups for global direct reclaim

[Michal Hocko]

On Wed 30-01-19 06:11:17, Yang Shi wrote:
> In current implementation, both kswapd and direct reclaim has to iterate
> all mem cgroups.  It is not a problem before offline mem cgroups could
> be iterated.  But, currently with iterating offline mem cgroups, it
> could be very time consuming.  In our workloads, we saw over 400K mem
> cgroups accumulated in some cases, only a few hundred are online memcgs.
> Although kswapd could help out to reduce the number of memcgs, direct
> reclaim still get hit with iterating a number of offline memcgs in some
> cases.  We experienced the responsiveness problems due to this
> occassionally.
> 
> A simple test with pref shows it may take around 220ms to iterate 8K memcgs
> in direct reclaim:
>              dd 13873 [011]   578.542919: vmscan:mm_vmscan_direct_reclaim_begin
>              dd 13873 [011]   578.758689: vmscan:mm_vmscan_direct_reclaim_end
> So for 400K, it may take around 11 seconds to iterate all memcgs.
> 
> Here just break the iteration once it reclaims enough pages as what
> memcg direct reclaim does.  This may hurt the fairness among memcgs.  But
> the cached iterator cookie could help to achieve the fairness more or
> less.
> 
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
> v2: Added some test data in the commit log
>     Updated commit log about iterator cookie could maintain fairness
>     Dropped !global_reclaim() since !current_is_kswapd() is good enough
> 
>  mm/vmscan.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a714c4f..5e35796 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2764,16 +2764,15 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>  				   sc->nr_reclaimed - reclaimed);
>  
>  			/*
> -			 * Direct reclaim and kswapd have to scan all memory
> -			 * cgroups to fulfill the overall scan target for the
> -			 * node.
> +			 * Kswapd have to scan all memory cgroups to fulfill
> +			 * the overall scan target for the node.
>  			 *
>  			 * Limit reclaim, on the other hand, only cares about
>  			 * nr_to_reclaim pages to be reclaimed and it will
>  			 * retry with decreasing priority if one round over the
>  			 * whole hierarchy is not sufficient.
>  			 */
> -			if (!global_reclaim(sc) &&
> +			if (!current_is_kswapd() &&
>  					sc->nr_reclaimed >= sc->nr_to_reclaim) {
>  				mem_cgroup_iter_break(root, memcg);
>  				break;
> -- 
> 1.8.3.1
> 

-- 
Michal Hocko
SUSE Labs