Re: [PATCH v3 0/9] mm: introduce Designated Movable Blocks
From: Doug Berger
Date: Wed Nov 02 2022 - 18:34:05 EST
On 10/26/2022 3:55 AM, Mel Gorman wrote:
On Thu, Oct 20, 2022 at 02:53:09PM -0700, Doug Berger wrote:
MOTIVATION:
Some Broadcom devices (e.g. 7445, 7278) contain multiple memory
controllers with each mapped in a different address range within
a Uniform Memory Architecture. Some users of these systems have
expressed the desire to locate ZONE_MOVABLE memory on each
memory controller to allow user space intensive processing to
make better use of the additional memory bandwidth.
Unfortunately, the historical monotonic layout of zones would
mean that if the lowest addressed memory controller contains
ZONE_MOVABLE memory then all of the memory available from
memory controllers at higher addresses must also be in the
ZONE_MOVABLE zone. This would force all kernel memory accesses
onto the lowest addressed memory controller and significantly
reduce the amount of memory available for non-movable
allocations.
I didn't review the first version of this patch because others, particularly
David Hildenbrand highlighted many of the concerns I had. I broadly followed
the discussion but didn't respond because I live in a permanent state of
having too much to do but with a new version, I have to say something.
I am familiar with that state and as a beneficiary of your hard work
I'll take the opportunity to say thanks and I appreciate you taking the
time to respond.
The three big questions he initially asked were
How large are these areas typically?
How large are they in comparison to other memory in the system?
How is this memory currently presented to the system?
Can you share some more how exactly ZONE_MOVABLE would help here to make
better use of the memory bandwidth?
Zones are about addressing limitations primarily and frankly, ZONE_MOVABLE
was a bad idea in retrospect. Today, the preferred approach would have
been to create a separate NUMA node with distance-1 to the local node
(fudge by adding 1 to the local distance "10" for zonelist purposes)
that was ZONE_MOVABLE with the zonelists structured such that GFP_MOVABLE
allocations would prefer the "movable" node first.
I'm afraid I don't completely follow what you are suggesting here.
While I don't recall
why I did not take that approach, it most likely was because CONFIG_NUMA
was not always set, it was only intended for hugetlbfs allocations and
maybe I didn't have the necessary skill or foresight to take that approach.
It remains true that CONFIG_NUMA is not always set and that is a key
motivator for this patch set. For example, Google is moving to a common
GKI kernel for their Google TV platform that they are requiring vendors
to support. Currently the arm64 GKI kernel does not set CONFIG_NUMA and
it seems unlikely that we will be able to get all vendors to accept such
a change.
Hotplugs requirements are somewhat different, the primary motivation that
I'm aware of is being able to guarantee they can be offlined, particularly
nodes, which can be done in some circumstances. Generally hotplug does
not care what uses the memory as long as it can be removed later. The
requirements for restricted access to high speed memory is different.
This is effectively the same requirement that an implementation of
'reusable' reserved memory has. A driver that owns reserved memory may
not care what uses the memory as long as the memory can be reclaimed
when the driver needs it. This is functionally analogous to memory
hotplug. Reserved memory that is 'reusable' and compatible with
'shared-dma-pool' uses the CMA implementation, but there is room for an
alternative implementation that shares the memory more aggressively.
This is a separate motivator for Designated Movable Block support, but I
am deferring that discussion since it is desirable to have a more
extended debate over APIs and such.
There is a high degree of uncertainity of how these regions are to be
used by applications to get access to the high speed memory, to quote
I'm not certain what is typical because these systems are highly
configurable and Broadcom's customers have different ideas about
application processing.
...
The Designated Movable Block concept introduced here has the
potential to offer useful services to different constituencies. I
tried to highlight this in my V1 patch set with the hope of
attracting some interest, but it can complicate the overall
discussion, so I would like to maybe narrow the discussion here. It
may be good to keep them in mind when assessing the overall value,
but perhaps the "other opportunities" can be covered as a follow
on discussion.
I note the "potential" part here because we don't actually know.
I used "potential" here not as in "it might be useful", but rather that
"different constituencies (i.e. people outside of the Broadcom
ecosystem) might also find them useful".
A
major limitation of ZONE_MOVABLE is that there is no way of controlling
access from userspace to restrict the high-speed memory to a designated
application, only to all applications in general. The primary interface
to control access to memory with different characteristics is mempolicies
which is NUMA orientated, not zone orientated. So, if there is a special
application that requires exclusive access, it's very difficult to configure
based on zones. Furthermore, page table pages mapping data located in the
high-speed region are stored in the slower memory which potentially impacts
the performance if the working set of the application exceeds TLB reach.
Finally, while there is mention that Broadcom may have some special
interface to determine what applications can use the high-speed region,
it's hardware-specific as opposed to something that belongs in the core mm.
I agree that keeping the high-speed memory in a local node and using "sticky"
pageblocks or CMA has limitations of its own but in itself, that does not
justify using ZONE_MOVABLE in my opinion. The statement that ARM can have
multiple controllers with equal distance and bandwidth (if I'm reading it
correctly) but places them in different zones.... that's just a bit weird if
there are no other addressing limitations. It's not obvious why ARM would do
that, but it also does not matter because it shouldn't be a core mm concern.
There appears to be some confusion regarding my explanation of multiple
memory controllers on a device like the BCM7278. There is no inherent
performance difference between the two memory controllers and their
attached DRAM. They merely provide the opportunity to perform memory
accesses in parallel for different physical address ranges. The physical
address ranges were selected by the SoC designers for reasons only known
to them, but I'm sure they had no consideration of zones in their
decision making. The selection of zones remains an artifact of the
design of Linux.
Since the BCM7278 contains a 4-core SMP cluster and each core can have
multiple outstanding memory transactions the speed of DDR transactions
can create a bottleneck for the system. If each memory controller has an
effective bandwidth of X then, provided the system memory accesses can
be distributed across both memory controllers, the combined effective
bandwidth can be additive (X + X = 2X). Of course the actual result is
highly dependent on the dependent clause "provided the system memory
accesses can be distributed across both memory controllers". The
accesses do not need to be evenly distributed to gain a benefit. We just
want to reduce any idle time on each memory controller.
It was observed that the monotonic zone layout for a non-NUMA system
(like this one) creates a bias for kernel space to use lower physical
memory addresses and user space to use higher physical memory addresses.
Broadcom customers requested the ability to locate movablecore memory
within the physical address range of each memory controller and reported
that it improved their system performance. Unfortunately, I do not have
access to their data and I doubt they would allow me to share it if I
did. I don't believe this is really about trying to optimize the
performance of a specific application as much as trying to prevent
overall system performance degradation from underutilized memory bandwidth.
There are already examples of where memory is physically "local" to
the CPU but has different bandwidth or latency including High Bandwidth
(HBM), Sub-NUMA Clustering (SNC), PMEM as a memory-life device and some
AMD EPYC Chips, particularly the first generation where a sockets memory
controllers had different distances. With the broadcom controllers,
it sounds like a local memory controller but the bandwidth available
differs. It's functionally equivalent to HBM.
The bandwidth available does not differ, but if too few transactions
target one of the memory controllers, that controllers bandwidth is
underutilized.
The fact that the memory access is physically local to the CPU socket is
irrelevant when the characteristics of that locality differs. NUMA stands
for Non-Uniform Memory Access and if bandwidth to different address ranges
differs, then the system is inherently NUMA even if that is inconvenient.
The bandwidth to different address ranges does not differ. A single
threaded application should see no performance difference regardless of
where its memory is located. However, if multiple application threads
are executing in parallel and the memory is divided between the memory
controllers they will be able to do more work per unit of time than if
the memory is predominantly located on one memory controller.
While I have not evaluated the implementation in detail, there is already
infrastructure dealing with tiered memory (memory that is local but has
different characteristics) with support for moving memory between tiers
depending on access patterns. Memory policies can be used to restrict
access to what processes can access the higher bandwidth memory. Given the
use case for DSM, I suspect that the intent is "application data uses high
bandwidth memory where possible and kernel uses lower bandwidth memory"
which is probably fine for an appliance because there is only one workload
but it's not a generic solution suitable.
Going back to the original questions;
How large are these areas typically?
How large are they in comparison to other memory in the system?
I am treating this as the same question because the consequencs are the
same. A high ratio of !MOVABLE:MOVABLE can cause big problems including
premature OOM, surprising reclaim behaviour etc
This is what makes the current movablecore implementation unacceptable.
In order to get any movablecore memory in the lower physical address
range requires all of the upper physical address range to be movablecore
which is horribly unbalanced. Specifying a value like
'movablecore=256M@2G,512M' with this patch set allows us to specify
512MB of total movablecore with 256MB of it at a physical address within
the lower memory controller and the remainder at the highest addresses
of the upper memory controller.
How is this memory currently presented to the system?
It's local, but with different characteristics so it's inherently NUMA
because it's Non-Uniform, there is no getting away from that.
It does not have different characteristics.
Can you share some more how exactly ZONE_MOVABLE would help here to make
better use of the memory bandwidth?
In the appliance case, it doesn't matter if the intent is that "all
application data should use high bandwidth memory where possible and
the application phase behaviour is predictable" and that may very well
work fine for the users of the Broadcom platforms with multiple memory
controllers. It does not work at all for the general where access must
be restricted to a subset of tasks in a general system that can only be
controlled with memory policies.
The high bandwidth memory should be representated as a NUMA node, optionally
to create that node as ZONE_MOVABLE and relying on the zonelists to select
the movable zone as the first preference.
This patch set is fundamentally about greater control over the placement
of movablecore memory. The current implementation of movablecore
requires all of the ZONE_MOVABLE memory to be located at the highest
physical addresses of the system when CONFIG_NUMA is not set. Allowing
the specification of a base address allows greater flexibility on
systems where there are benefits.
Thanks again for your time,
Doug