Re: [PATCH RFC] fs: drop_caches: introduce per-node drop_caches interface

[Michal Hocko <mhocko@suse.com>]

On Thu 09-04-26 14:35:03, Kefeng Wang wrote:
> Add a sysfs interface at /sys/devices/system/node/nodeX/drop_caches
> to allow dropping caches on a specific NUMA node.
> 
> The existing global drop_caches mechanism (/proc/sys/vm/drop_caches)
> operates across all NUMA nodes indiscriminately, causing,
> - Unnecessary eviction of hot cache on some nodes
> - Performance degradation for applications with NUMA affinity
> - Long times spent on large systems with lots of memory
> 
> By exposing a per-node interface, admistrator can,
> - Target specific nodes experiencing memory pressure
> - Preserve cache on unaffected nodes
> - Perform reclamation with finer granularity

Quite honestly drop_caches is not the best interface to build any new
functionality on top of. It has backfired a lot in the past and we have
tried to make it extra clear that this should be used for debugging
purposes only. Extending it further sounds like a bad step.

> One use cases is hot-pluggable NUMA nodes, during hot-remove, simply
> dropping pagecache is far more efficient than migrating large amounts
> of pages to other nodes, which also eliminating the risk of accessing
> potentially faulty memory.

Does the per-node reclaim interface can help with this by
any means?

-- 
Michal Hocko
SUSE Labs