linux-kernel - Re: [RFC] Memory Tiering

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Message-ID: <0679872d-3d03-2fa3-5bd2-80f694357203@redhat.com>
Date:   Thu, 17 Oct 2019 10:07:07 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>, Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "Verma, Vishal L" <vishal.l.verma@...el.com>,
        Wu Fengguang <fengguang.wu@...el.com>,
        Huang Ying <ying.huang@...el.com>
Subject: Re: [RFC] Memory Tiering

On 16.10.19 22:05, Dave Hansen wrote:
> The memory hierarchy is getting more complicated and the kernel is
> playing an increasing role in managing the different tiers.  A few
> different groups of folks described "migration" optimizations they were
> doing in this area at LSF/MM earlier this year.  One of the questions
> folks asked was why autonuma wasn't being used.
> 
> At Intel, the primary new tier that we're looking at is persistent
> memory (PMEM).  We'd like to be able to use "persistent memory"
> *without* using its persistence properties, treating it as slightly
> slower DRAM.  Keith Busch has some patches to use NUMA migration to
> automatically migrate DRAM->PMEM instead of discarding it near the end
> of the reclaim process.  Huang Ying has some patches which use a
> modified autonuma to migrate frequently-used data *back* from PMEM->DRAM.

Very interesting topic. I heard similar demand from HPC folks 
(especially involving other memory types ("tiers")). There, I think you 
often want to let the application manage that. But of course, for many 
applications an automatic management might already be beneficial.

Am I correct that you are using PMEM in this area along with ZONE_DEVICE 
and not by giving PMEM to the buddy (add_memory())?

> 
> We've tried to do this all generically so that it is not tied to
> persistent memory and can be applied to any memory types in lots of
> topologies.
> 
> We've been running this code in various forms for the past few months,
> comparing it to pure DRAM and hardware-based caching.  The initial
> results are encouraging and we thought others might want to take a look
> at the code or run their own experiments.  We're expecting to post the
> individual patches soon.  But, until then, the code is available here:
> 
>   	https://git.kernel.org/pub/scm/linux/kernel/git/vishal/tiering.git
> 
> and is tagged with "tiering-0.2", aka. d8e31e81b1dca9.
> 
> Note that internally folks have been calling this "hmem" which is
> terribly easy to confuse with the existing hmm.  There are still some
> "hmem"'s in the tree, but I don't expect them to live much longer.
> 


-- 

Thanks,

David / dhildenb