Re: [PATCH v1] mm: disable top-tier fallback to reclaim on proactive reclaim

[Andrew Morton]

On Thu,  1 Dec 2022 15:33:17 -0800 Mina Almasry <almasrymina@xxxxxxxxxx> wrote:

> Reclaiming directly from top tier nodes breaks the aging pipeline of
> memory tiers.  If we have a RAM -> CXL -> storage hierarchy, we
> should demote from RAM to CXL and from CXL to storage. If we reclaim
> a page from RAM, it means we 'demote' it directly from RAM to storage,
> bypassing potentially a huge amount of pages colder than it in CXL.
> 
> However disabling reclaim from top tier nodes entirely would cause ooms
> in edge scenarios where lower tier memory is unreclaimable for whatever
> reason, e.g. memory being mlocked() or too hot to reclaim.  In these
> cases we would rather the job run with a performance regression rather
> than it oom altogether.
> 
> However, we can disable reclaim from top tier nodes for proactive reclaim.
> That reclaim is not real memory pressure, and we don't have any cause to
> be breaking the aging pipeline.
> 

Is this purely from code inspection, or are there quantitative
observations to be shared?