linux-kernel - Re: [PATCH 02/10] mm/numa: automatically generate node migration order

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Date:   Thu, 15 Apr 2021 08:35:09 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Wei Xu <weixugc@...gle.com>, Oscar Salvador <osalvador@...e.de>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Yang Shi <shy828301@...il.com>,
        David Rientjes <rientjes@...gle.com>,
        Huang Ying <ying.huang@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 02/10] mm/numa: automatically generate node migration
 order

On 4/14/21 9:07 PM, Wei Xu wrote:
> On Wed, Apr 14, 2021 at 1:08 AM Oscar Salvador <osalvador@...e.de> wrote:
>> Fast class/memory are pictured as those nodes with CPUs, while Slow class/memory
>> are PMEM, right?
>> Then, what stands for medium class/memory?
> 
> That is Dave's example.  I think David's guess makes sense (HBM - fast, DRAM -
> medium, PMEM - slow).  It may also be possible that we have DDR5 as fast,
> CXL-DDR4 as medium, and CXL-PMEM as slow.  But the most likely use cases for
> now should be just two tiers: DRAM vs PMEM or other types of slower
> memory devices.

Yes, it would be nice to apply this to fancier tiering systems.  But
DRAM/PMEM combos are out in the wild today and it's where I expect this
to be used first.

> This can help enable more flexible demotion policies to be
> configured, such as to allow a cgroup to allocate from all fast tier
> nodes, but only demote to a local slow tier node.  Such a policy can
> reduce memory stranding at the fast tier (compared to if memory
> hardwall is used) and still allow demotion from all fast tier nodes
> without incurring the expensive random accesses to the demoted pages
> if they were demoted to remote slow tier nodes.

Could you explain this stranding effect in a bit more detail?  I'm not
quite following.