linux-kernel - Re: Is it possible to implement the per-node page cache for programs/libraries?

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Date:   Mon, 6 Sep 2021 09:56:17 +0000
From:   Huang Shijie <shijie@...amperecomputing.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Shijie Huang <shijie@...eremail.onmicrosoft.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Frank Wang <zwang@...erecomputing.com>
Subject: Re: Is it possible to implement the per-node page cache for
 programs/libraries?

Hi Linus,
On Fri, Sep 03, 2021 at 12:08:03PM -0700, Linus Torvalds wrote:
> On Fri, Sep 3, 2021 at 12:02 PM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > Was there a reason you chose to do it that way instead of having per-node
> > i_mapping pointers?
> 
> You can't have per-node i_mapping pointers without huge coherence issues.
> 
> If you don't care about coherence, that's fine - but that has to be a
> user-space decision (ie "I will just replicate this file").
> 
> You can't just have the kernel decide "I'll map this set of pages on
> this node, and that other ser of pages on that other node", in case
> there's MAP_SHARED things going on.
> 
> Anyway, I think very fundamentally this is one of those things where
> 99.9% of all people don't care, and DO NOT WANT the complexity.
> 
> And the 0.1% that _does_ care really could and should do this in user
> space, because they know they care.
> 
> Asking the kernel to do complex things in critical core functions for
> something that is very very rare and irrelevant to most people, and
> that can and should just be done in user space for the people who care
> is the wrong approach.
> 
> Because the question here really should be "is this truly important,
> and does this need kernel help because user space simply cannot do it
> itself".
> 
> And the answer is a fairly simple "no".
Okay.

Thanks for confirming this.

Thanks
Huang Shijie