linux-kernel - Re: [RFC] memory tiering: use small chunk size and more tiers

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Date:   Wed, 2 Nov 2022 09:39:25 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Bharata B Rao <bharata@....com>,
        Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alistair Popple <apopple@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Jagdish Gediya <jvgediya.oss@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Tim Chen <tim.c.chen@...el.com>, Wei Xu <weixugc@...gle.com>,
        Yang Shi <shy828301@...il.com>
Subject: Re: [RFC] memory tiering: use small chunk size and more tiers

On Wed 02-11-22 16:28:08, Huang, Ying wrote:
> Michal Hocko <mhocko@...e.com> writes:
> 
> > On Wed 02-11-22 16:02:54, Huang, Ying wrote:
> >> Michal Hocko <mhocko@...e.com> writes:
> >> 
> >> > On Wed 02-11-22 08:39:49, Huang, Ying wrote:
> >> >> Michal Hocko <mhocko@...e.com> writes:
> >> >> 
> >> >> > On Mon 31-10-22 09:33:49, Huang, Ying wrote:
> >> >> > [...]
> >> >> >> In the upstream implementation, 4 tiers are possible below DRAM.  That's
> >> >> >> enough for now.  But in the long run, it may be better to define more.
> >> >> >> 100 possible tiers below DRAM may be too extreme.
> >> >> >
> >> >> > I am just curious. Is any configurations with more than couple of tiers
> >> >> > even manageable? I mean applications have been struggling even with
> >> >> > regular NUMA systems for years and vast majority of them is largerly
> >> >> > NUMA unaware. How are they going to configure for a more complex system
> >> >> > when a) there is no resource access control so whatever you aim for
> >> >> > might not be available and b) in which situations there is going to be a
> >> >> > demand only for subset of tears (GPU memory?) ?
> >> >> 
> >> >> Sorry for confusing.  I think that there are only several (less than 10)
> >> >> tiers in a system in practice.  Yes, here, I suggested to define 100 (10
> >> >> in the later text) POSSIBLE tiers below DRAM.  My intention isn't to
> >> >> manage a system with tens memory tiers.  Instead, my intention is to
> >> >> avoid to put 2 memory types into one memory tier by accident via make
> >> >> the abstract distance range of each memory tier as small as possible.
> >> >> More possible memory tiers, smaller abstract distance range of each
> >> >> memory tier.
> >> >
> >> > TBH I do not really understand how tweaking ranges helps anything.
> >> > IIUC drivers are free to assign any abstract distance so they will clash
> >> > without any higher level coordination.
> >> 
> >> Yes.  That's possible.  Each memory tier corresponds to one abstract
> >> distance range.  The larger the range is, the higher the possibility of
> >> clashing is.  So I suggest to make the abstract distance range smaller
> >> to reduce the possibility of clashing.
> >
> > I am sorry but I really do not understand how the size of the range
> > actually addresses a fundamental issue that each driver simply picks
> > what it wants. Is there any enumeration defining basic characteristic of
> > each tier? How does a driver developer knows which tear to assign its
> > driver to?
> 
> The smaller range size will not guarantee anything.  It just tries to
> help the default behavior.
> 
> The drivers are expected to assign the abstract distance based on the
> memory latency/bandwidth, etc.

Would it be possible/feasible to have a canonical way to calculate the
abstract distance from these characteristics by the core kernel so that
drivers do not even have fall into that trap?

-- 
Michal Hocko
SUSE Labs