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From:   SeongJae Park <sj@kernel.org>
To:     Xin Hao <xhao@linux.alibaba.com>
Cc:     SeongJae Park <sj@kernel.org>, rongwei.wang@linux.alibaba.com,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, rientjes@google.com,
        linux-damon@amazon.com
Subject: Re: [RFC PATCH V1 0/5] mm/damon: Add NUMA access statistics function support
Date:   Fri, 18 Feb 2022 08:03:40 +0000
Message-Id: <20220218080340.11566-1-sj@kernel.org>
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On Fri, 18 Feb 2022 10:21:27 +0800 Xin Hao <xhao@linux.alibaba.com> wrote:

> Hi SeongJae:
> 
> On 2/17/22 4:29 PM, SeongJae Park wrote:
> > + David Rientjes, who has shown interest[1] in this topic.
> >
> > [1] https://lore.kernel.org/linux-mm/bcc8d9a0-81d-5f34-5e4-fcc28eb7ce@google.com/
> >
> > ---
> >
> > Hi Xin,
> >
> >
> > Thank you always for great patches!
> >
> > On Wed, 16 Feb 2022 16:30:36 +0800 Xin Hao <xhao@linux.alibaba.com> wrote:
> >
[...]
> > I'd like to comment on the high level design at the moment.  To my
> > understanding, this patchset extends DAMON core and monitoring operations for
> > virtual address spaces (vaddr) and the physical address space (paddr) to
> > monitor NUMA-local/remote accesses via PROT_NONE and page faults mechanism.
> >
> > The underlying mechanism for NUMA-local/remote accesses (PROT_NONE and page
> > fault) looks ok to me.  But, changes to the core and vaddr/paddr operations
> > looks unnecessary, to me.  That's also not for general use cases.
> You are right, adding NUMA access statistics does make the PA & VA codes 
> look confusing。
> >
> > I think it would be simpler to implment more monitoring operations for NUMA
> > monitoring use case (one for NUMA-local accesses accounting and another one for
> > NUMA-remote accesses accounting), alongside vaddr and paddr.  Then, users could
> > configure DAMON to have three monitoring contexts (one with vaddr ops, second
> > one with numa-local ops, and third one with numa-remote ops), run those
> > concurrently, then show the three results and make some decisions like
> > migrations.
> 
> Thanks for your advice, I will implement these in the next version, But 
> from my understanding or maybe
> 
> I didn't get what you were thinking, I think only one monitor context is 
> needed for NUMA Local & Remote,
> 
> Do not need a separate implementation like "numa_local_ops" and 
> "numa_remote_ops", just set "numa_access_ops" is ok.

Sorry for insufficient explanation of my concern.  In short, I'm concerning
about the regions adjustment.

You may do so by storing NUMA-local access count and NUMA-remote access
count in the nr_acceses filed of each region, e.g., saving NUMA-local access
count in upper-half bits of nr_accesses and saving NUMA-remote access count in
the lower-half bits.  However, then DAMON will do the regions adjustment based
on the NUMA-local/remote accesses count mixed value, so the accuracy would be
degraded.  So I think we need to implement each monitoring operations set for
each accesses that we want to monitor.

> 
> >
> > One additional advantage of this approach is that the accuracy for
> > NUMA-local/remote accessed could be better, because the contexts configured to
> > use NUMA-local/remote monitoring ops will do the regions adjustment with
> > NUMA-local/remote accesses (to my understanding, this patchset let regions have
> > NUMA-local/remote accesses counter in addition to the total one, but still use
> > only the total one for the regions adjustment).

My previous comment above might help clarifying my concern.

If I'm missing something, please let me know.


Thanks,
SJ

> >
> > If I'm missing something, please let me know.
> >
> >
> > Thanks,
> > SJ
> >
> >> --
> >> 2.27.0
> 
> -- 
> Best Regards!
> Xin Hao
>