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

[Tim Chen <tim.c.chen@linux.intel.com>]

On 6/18/21 4:59 PM, Shakeel Butt wrote:
> On Fri, Jun 18, 2021 at 3:11 PM Tim Chen <tim.c.chen@linux.intel.com> wrote:
>>
>>
>>
>> On 6/17/21 11:48 AM, Shakeel Butt wrote:
> [...]
>>>
>>> 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).
>>
>> Probably we could first start with accounting the memory used in each
>> NUMA node for a cgroup and exposing this information to user space.
>> I think that is useful regardless.
>>
> 
> Is memory.numa_stat not good enough? 

Yeah, forgot numa_stat is already there.  Thanks for reminding me.

> This interface does miss
> __GFP_ACCOUNT non-slab allocations, percpu and sock.

numa_stat should be good enough for now.

> 
>> There is still a question of whether we want to define a set of
>> numa node or tier and extend the accounting and management at that
>> memory tier abstraction level.
>>
> [...]
>>>
>>> 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
>>> 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.
>>
>> If I understand what you are saying is you desire the kernel to provide
>> the interface to expose performance information like
>> "X% of accesses of high priority jobs is from high tier",
> 
> I think we can estimate "X% of accesses to high tier" using existing
> perf/PMU counters. So, no new interface.

Using a perf counter will be okay to do for user space daemon, but I
think there will be objections from people that the kernel 
take away a perf counter to collect perf data in kernel.

Tim