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

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Message-ID: <027AE219-8C81-47DC-A241-4209C3F656A0@nvidia.com>
Date:   Tue, 16 Apr 2019 12:12:33 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Dave Hansen <dave.hansen@...el.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 16 Apr 2019, at 11:55, Dave Hansen wrote:

> 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.

I definitely agree. I also want to make the point that we might
want to improve page migration as well to show that demotion via
page migration will work. Since most of proposed demotion approaches
use the same page replacement policy as swapping, if we do not have
high-throughput page migration, we might draw false conclusions that
demotion is no better than swapping but demotion can actually do
much better. :)

--
Best Regards,
Yan Zi

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