linux-kernel - Re: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node

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Date:   Tue, 16 Apr 2019 08:55:41 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Zi Yan <ziy@...dia.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        mgorman@...hsingularity.net, riel@...riel.com, hannes@...xchg.org,
        akpm@...ux-foundation.org, keith.busch@...el.com,
        dan.j.williams@...el.com, fengguang.wu@...el.com, fan.du@...el.com,
        ying.huang@...el.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node

On 4/16/19 8:33 AM, Zi Yan wrote:
>> We have a reasonable argument that demotion is better than
>> swapping. So, we could say that even if a VMA has a strict NUMA
>> policy, demoting pages mapped there pages still beats swapping
>> them or tossing the page cache.  It's doing them a favor to
>> demote them.
> I just wonder whether page migration is always better than
> swapping, since SSD write throughput keeps improving but page
> migration throughput is still low. For example, my machine has a
> SSD with 2GB/s writing throughput but the throughput of 4KB page
> migration is less than 1GB/s, why do we want to use page migration
> for demotion instead of swapping?

Just because we observe that page migration apparently has lower
throughput today doesn't mean that we should consider it a dead end.