RE: [PATCH v3 0/3] create sysfs representation of ACPI HMAT

[Kogut, Jaroslaw]

> ... first thinking about redesigning the NUMA for
> heterogeneous memory may not be a good idea. Will look into this further.

I agree with comment that first a direction should be defined how to handle heterogeneous memory system.

> https://linuxplumbersconf.org/2017/ocw//system/presentations/4656/original/
> Hierarchical_NUMA_Design_Plumbers_2017.pdf

I miss in the presentation a user perspective of the new approach, e.g.
- How does application developer see/understand the heterogeneous memory system?
- How does app developer use the heterogeneous memory system? 
- What are modification in API/sys interfaces?

In other hand, if we assume that separate memory NUMA node has different memory capabilities/attributes from stand point of particular CPU, it is easy to explain for user how to describe/handle heterogeneous memory. 

Of course, current numa design is not sufficient in kernel in following areas today:
- Exposing memory attributes that describe heterogeneous memory system
- Interfaces to use the heterogeneous memory system, e.g. more sophisticated policies
- Internal mechanism in memory management, e.g. automigration, maybe something else.

> -----Original Message-----
> From: Anshuman Khandual [mailto:khandual@xxxxxxxxxxxxxxxxxx]
> Sent: Friday, December 22, 2017 4:10 AM
> To: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
> Cc: Anaczkowski, Lukasz <lukasz.anaczkowski@xxxxxxxxx>; Box, David E
> <david.e.box@xxxxxxxxx>; Kogut, Jaroslaw <Jaroslaw.Kogut@xxxxxxxxx>; Koss,
> Marcin <marcin.koss@xxxxxxxxx>; Koziej, Artur <artur.koziej@xxxxxxxxx>;
> Lahtinen, Joonas <joonas.lahtinen@xxxxxxxxx>; Moore, Robert
> <robert.moore@xxxxxxxxx>; Nachimuthu, Murugasamy
> <murugasamy.nachimuthu@xxxxxxxxx>; Odzioba, Lukasz
> <lukasz.odzioba@xxxxxxxxx>; Wysocki, Rafael J <rafael.j.wysocki@xxxxxxxxx>;
> Rafael J. Wysocki <rjw@xxxxxxxxxxxxx>; Schmauss, Erik
> <erik.schmauss@xxxxxxxxx>; Verma, Vishal L <vishal.l.verma@xxxxxxxxx>;
> Zheng, Lv <lv.zheng@xxxxxxxxx>; Andrew Morton <akpm@linux-
> foundation.org>; Balbir Singh <bsingharora@xxxxxxxxx>; Brice Goglin
> <brice.goglin@xxxxxxxxx>; Williams, Dan J <dan.j.williams@xxxxxxxxx>;
> Hansen, Dave <dave.hansen@xxxxxxxxx>; Jerome Glisse <jglisse@xxxxxxxxxx>;
> John Hubbard <jhubbard@xxxxxxxxxx>; Len Brown <lenb@xxxxxxxxxx>; Tim
> Chen <tim.c.chen@xxxxxxxxxxxxxxx>; devel@xxxxxxxxxx; linux-
> acpi@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; linux-nvdimm@xxxxxxxxxxxx
> Subject: Re: [PATCH v3 0/3] create sysfs representation of ACPI HMAT
> 
> On 12/14/2017 07:40 AM, Ross Zwisler wrote:
> > ==== Quick Summary ====
> >
> > Platforms exist today which have multiple types of memory attached to
> > a single CPU.  These disparate memory ranges have some characteristics
> > in common, such as CPU cache coherence, but they can have wide ranges
> > of performance both in terms of latency and bandwidth.
> 
> Right.
> 
> >
> > For example, consider a system that contains persistent memory,
> > standard DDR memory and High Bandwidth Memory (HBM), all attached to
> the same CPU.
> > There could potentially be an order of magnitude or more difference in
> > performance between the slowest and fastest memory attached to that CPU.
> 
> Right.
> 
> >
> > With the current Linux code NUMA nodes are CPU-centric, so all the
> > memory attached to a given CPU will be lumped into the same NUMA node.
> > This makes it very difficult for userspace applications to understand
> > the performance of different memory ranges on a given CPU.
> 
> Right but that might require fundamental changes to the NUMA
> representation.
> Plugging those memory as separate NUMA nodes, identify them through sysfs
> and try allocating from it through mbind() seems like a short term solution.
> 
> Though if we decide to go in this direction, sysfs interface or something similar
> is required to enumerate memory properties.
> 
> >
> > We solve this issue by providing userspace with performance
> > information on individual memory ranges.  This performance information
> > is exposed via
> > sysfs:
> >
> >   # grep . mem_tgt2/* mem_tgt2/local_init/* 2>/dev/null
> >   mem_tgt2/firmware_id:1
> >   mem_tgt2/is_cached:0
> >   mem_tgt2/local_init/read_bw_MBps:40960
> >   mem_tgt2/local_init/read_lat_nsec:50
> >   mem_tgt2/local_init/write_bw_MBps:40960
> >   mem_tgt2/local_init/write_lat_nsec:50
> 
> I might have missed discussions from earlier versions, why we have this kind of
> a "source --> target" model ? We will enlist properties for all possible "source --
> > target" on the system ? Right now it shows only bandwidth and latency
> properties, can it accommodate other properties as well in future ?
> 
> >
> > This allows applications to easily find the memory that they want to use.
> > We expect that the existing NUMA APIs will be enhanced to use this new
> > information so that applications can continue to use them to select
> > their desired memory.
> 
> I had presented a proposal for NUMA redesign in the Plumbers Conference this
> year where various memory devices with different kind of memory attributes
> can be represented in the kernel and be used explicitly from the user space.
> Here is the link to the proposal if you feel interested. The proposal is very
> intrusive and also I dont have a RFC for it yet for discussion here.
> 
> https://linuxplumbersconf.org/2017/ocw//system/presentations/4656/original/
> Hierarchical_NUMA_Design_Plumbers_2017.pdf
> 
> Problem is, designing the sysfs interface for memory attribute detection from
> user space without first thinking about redesigning the NUMA for
> heterogeneous memory may not be a good idea. Will look into this further.

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