Re: [LSF/MM TOPIC] Tiered memory accounting and management

[Yang Shi <shy828301@gmail.com>]

On Thu, Jun 17, 2021 at 11:49 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> Thanks Yang for the CC.
>
> On Tue, Jun 15, 2021 at 5:17 PM Yang Shi <shy828301@gmail.com> wrote:
> >
> > On Mon, Jun 14, 2021 at 2:51 PM Tim Chen <tim.c.chen@linux.intel.com> wrote:
> > >
> > >
> > > From: Tim Chen <tim.c.chen@linux.intel.com>
> > >
> > > Tiered memory accounting and management
> > > ------------------------------------------------------------
> > > Traditionally, all RAM is DRAM.  Some DRAM might be closer/faster
> > > than others, but a byte of media has about the same cost whether it
> > > is close or far.  But, with new memory tiers such as High-Bandwidth
> > > Memory or Persistent Memory, there is a choice between fast/expensive
> > > and slow/cheap.  But, the current memory cgroups still live in the
> > > old model. There is only one set of limits, and it implies that all
> > > memory has the same cost.  We would like to extend memory cgroups to
> > > comprehend different memory tiers to give users a way to choose a mix
> > > between fast/expensive and slow/cheap.
> > >
> > > To manage such memory, we will need to account memory usage and
> > > impose limits for each kind of memory.
> > >
> > > There were a couple of approaches that have been discussed previously to partition
> > > the memory between the cgroups listed below.  We will like to
> > > use the LSF/MM session to come to a consensus on the approach to
> > > take.
> > >
> > > 1.      Per NUMA node limit and accounting for each cgroup.
> > > We can assign higher limits on better performing memory node for higher priority cgroups.
> > >
> > > There are some loose ends here that warrant further discussions:
> > > (1) A user friendly interface for such limits.  Will a proportional
> > > weight for the cgroup that translate to actual absolute limit be more suitable?
> > > (2) Memory mis-configurations can occur more easily as the admin
> > > has a much larger number of limits spread among between the
> > > cgroups to manage.  Over-restrictive limits can lead to under utilized
> > > and wasted memory and hurt performance.
> > > (3) OOM behavior when a cgroup hits its limit.
> > >
>
> This (numa based limits) is something I was pushing for but after
> discussing this internally with userspace controller devs, I have to
> backoff from this position.
>
> The main feedback I got was that setting one memory limit is already
> complicated and having to set/adjust these many limits would be
> horrifying.

Yes, that is also what I heard of.

>
> > > 2.      Per memory tier limit and accounting for each cgroup.
> > > We can assign higher limits on memories in better performing
> > > memory tier for higher priority cgroups.  I previously
> > > prototyped a soft limit based implementation to demonstrate the
> > > tiered limit idea.
> > >
> > > There are also a number of issues here:
> > > (1)     The advantage is we have fewer limits to deal with simplifying
> > > configuration. However, there are doubts raised by a number
> > > of people on whether we can really properly classify the NUMA
> > > nodes into memory tiers. There could still be significant performance
> > > differences between NUMA nodes even for the same kind of memory.
> > > We will also not have the fine-grained control and flexibility that comes
> > > with a per NUMA node limit.
> > > (2)     Will a memory hierarchy defined by promotion/demotion relationship between
> > > memory nodes be a viable approach for defining memory tiers?
> > >
> > > These issues related to  the management of systems with multiple kind of memories
> > > can be ironed out in this session.
> >
> > Thanks for suggesting this topic. I'm interested in the topic and
> > would like to attend.
> >
> > Other than the above points. I'm wondering whether we shall discuss
> > "Migrate Pages in lieu of discard" as well? Dave Hansen is driving the
> > development and I have been involved in the early development and
> > review, but it seems there are still some open questions according to
> > the latest review feedback.
> >
> > Some other folks may be interested in this topic either, CC'ed them in
> > the thread.
> >
>
> At the moment "personally" I am more inclined towards a passive
> approach towards the memcg accounting of memory tiers. By that I mean,
> let's start by providing a 'usage' interface and get more
> production/real-world data to motivate the 'limit' interfaces. (One
> minor reason is that defining the 'limit' interface will force us to
> make the decision on defining tiers i.e. numa or a set of numa or
> others).
>
> IMHO we should focus more on the "aging" of the application memory and
> "migration/balance" between the tiers. I don't think the memory
> reclaim infrastructure is the right place for these operations
> (unevictable pages are ignored and not accurate ages). What we need is

Why is unevictable pages a problem? I don't get why you have to demote
unevictable pages. If you do care what nodes the memory will be
mlock'ed on, don't you have to move the memory to the target nodes
before mlock them?

> proactive continuous aging and balancing. We need something like, with
> additions, Multi-gen LRUs or DAMON or page idle tracking for aging and
> a new mechanism for balancing which takes ages into account.

I agree the better balance could be reached by more accurate aging. It
is a more general problem than tier'ed memory specific.

>
> To give a more concrete example: Let's say we have a system with two
> memory tiers and multiple low and high priority jobs. For high
> priority jobs, set the allocation try list from high to low tier and
> for low priority jobs the reverse of that (I am not sure if we can do
> that out of the box with today's kernel). In the background we migrate

AFAICT, I don't think we have. With the current APIs, you just can
bind to a set of nodes, but the fallback order is one way.

> cold memory down the tiers and hot memory in the reverse direction.
>
> In this background mechanism we can enforce all different limiting
> policies like Yang's original high and low tier percentage or
> something like X% of accesses of high priority jobs should be from
> high tier. Basically I am saying until we find from production data
> that this background mechanism is not strong enough to enforce passive
> limits, we should delay the decision on limit interfaces.