Re: [RFC PATCH v1 00/11] Manage the top tier memory in a tiered memory

[Jonathan Cameron]

On Mon, 12 Apr 2021 12:20:22 -0700
Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:

> On Fri, Apr 9, 2021 at 4:26 PM Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote:
> >
> >
> > On 4/8/21 4:52 AM, Michal Hocko wrote:
> >  
> > >> The top tier memory used is reported in
> > >>
> > >> memory.toptier_usage_in_bytes
> > >>
> > >> The amount of top tier memory usable by each cgroup without
> > >> triggering page reclaim is controlled by the
> > >>
> > >> memory.toptier_soft_limit_in_bytes  
> > >  
> >
> > Michal,
> >
> > Thanks for your comments.  I will like to take a step back and
> > look at the eventual goal we envision: a mechanism to partition the
> > tiered memory between the cgroups.
> >
> > A typical use case may be a system with two set of tasks.
> > One set of task is very latency sensitive and we desire instantaneous
> > response from them. Another set of tasks will be running batch jobs
> > were latency and performance is not critical.   In this case,
> > we want to carve out enough top tier memory such that the working set
> > of the latency sensitive tasks can fit entirely in the top tier memory.
> > The rest of the top tier memory can be assigned to the background tasks.
> >
> > To achieve such cgroup based tiered memory management, we probably want
> > something like the following.
> >
> > For generalization let's say that there are N tiers of memory t_0, t_1 ... t_N-1,
> > where tier t_0 sits at the top and demotes to the lower tier.
> > We envision for this top tier memory t0 the following knobs and counters
> > in the cgroup memory controller
> >
> > memory_t0.current       Current usage of tier 0 memory by the cgroup.
> >
> > memory_t0.min           If tier 0 memory used by the cgroup falls below this low
> >                         boundary, the memory will not be subjected to demotion
> >                         to lower tiers to free up memory at tier 0.
> >
> > memory_t0.low           Above this boundary, the tier 0 memory will be subjected
> >                         to demotion.  The demotion pressure will be proportional
> >                         to the overage.
> >
> > memory_t0.high          If tier 0 memory used by the cgroup exceeds this high
> >                         boundary, allocation of tier 0 memory by the cgroup will
> >                         be throttled. The tier 0 memory used by this cgroup
> >                         will also be subjected to heavy demotion.
> >
> > memory_t0.max           This will be a hard usage limit of tier 0 memory on the cgroup.
> >
> > If needed, memory_t[12...].current/min/low/high for additional tiers can be added.
> > This follows closely with the design of the general memory controller interface.
> >
> > Will such an interface looks sane and acceptable with everyone?
> >  
> 
> I have a couple of questions. Let's suppose we have a two socket
> system. Node 0 (DRAM+CPUs), Node 1 (DRAM+CPUs), Node 2 (PMEM on socket
> 0 along with Node 0) and Node 3 (PMEM on socket 1 along with Node 1).
> Based on the tier definition of this patch series, tier_0: {node_0,
> node_1} and tier_1: {node_2, node_3}.
> 
> My questions are:
> 
> 1) Can we assume that the cost of access within a tier will always be
> less than the cost of access from the tier? (node_0 <-> node_1 vs
> node_0 <-> node_2)

No in large systems even it we can make this assumption in 2 socket ones.

> 2) If yes to (1), is that assumption future proof? Will the future
> systems with DRAM over CXL support have the same characteristics?
> 3) Will the cost of access from tier_0 to tier_1 be uniform? (node_0
> <-> node_2 vs node_0 <-> node_3). For jobs running on node_0, node_3
> might be third tier and similarly for jobs running on node_1, node_2
> might be third tier.
> 
> The reason I am asking these questions is that the statically
> partitioning memory nodes into tiers will inherently add platform
> specific assumptions in the user API.

Absolutely agree.

> 
> Assumptions like:
> 1) Access within tier is always cheaper than across tier.
> 2) Access from tier_i to tier_i+1 has uniform cost.
> 
> The reason I am more inclined towards having numa centric control is
> that we don't have to make these assumptions. Though the usability
> will be more difficult. Greg (CCed) has some ideas on making it better
> and we will share our proposal after polishing it a bit more.
> 

Sounds good, will look out for that.

Jonathan