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SubjectRe: [RFC v2 PATCH] mm: vmscan: do not iterate all mem cgroups for global direct reclaim
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@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> 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

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