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Date:   Wed, 1 Sep 2021 04:25:01 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Shijie Huang <shijie@amperemail.onmicrosoft.com>
Cc:     torvalds@linux-foundation.org, viro@zeniv.linux.org.uk,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        song.bao.hua@hisilicon.com, linux-kernel@vger.kernel.org,
        Frank Wang <zwang@amperecomputing.com>
Subject: Re: Is it possible to implement the per-node page cache for
 programs/libraries?
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On Wed, Sep 01, 2021 at 11:07:41AM +0800, Shijie Huang wrote:
>     In the NUMA, we only have one page cache for each file. For the
> program/shared libraries, the
> remote-access delays longer then the  local-access.
> 
> So, is it possible to implement the per-node page cache for
> programs/libraries?

At this point, we have no way to support text replication within a
process.  So what you're suggesting (if implemented) would work for
processes which limit themselves to a single node.  That is, if you
have a system with CPUs 0-3 on node 0 and CPUs 4-7 on node 1, a process
which only works on node 0 or only works on node 1 will get text on the
appropriate node.

If there's a process which runs on both nodes 0 and 1, there's no support
for per-node PGDs.  So it will get a mix of pages from nodes 0 and 1,
and that doesn't necessarily seem like a big win.  I haven't yet dived
into how hard it would be to make mm->pgd a per-node allocation.

I have been thinking about this a bit; one of our internal performance
teams flagged the potential performance win to me a few months ago.
I don't have a concrete design for text replication yet; there have been
various attempts over the years, but none were particularly compelling.

By the way, the degree of performance win varies between different CPUs,
but it's measurable on all the systems we've tested on (from three
different vendors).