Re: [RFC 0/4] RFC - Coherent Device Memory (Not for inclusion)

[Christoph Lameter <cl@linux.com>]

On Thu, 20 Apr 2017, Balbir Singh wrote:
> Couple of things are needed
>
> 1. Isolation of allocation

cgroups, memory policy and cpuset provide that

> 2. Isolation of certain algorithms like kswapd/auto-numa balancing

Ok that may mean adding some generic functionality to limit those

> > The approach sounds pretty invasive to me.
>
> Could you please elaborate, you mean the user space programming bits?

No I mean the modification of the memory policies in particular. We are
adding more exceptions to an already complex and fragile system.

Can we do this in a generic way just using hotplug nodes and some of the
existing isolation mechanisms?


> Ideally we need the following:
>
> 1. Transparency about being able to allocate memory anywhere and the ability
> to migrate memory between coherent device memory and normal system memory

If it is a memory node then you have that already.

> 2. The ability to explictly allocate memory from coherent device memory

Ditto

> 3. Isolation of normal allocations from coherent device memory unless
> explictly stated, same as (2) above

memory policies etc do that.

> 4. The ability to hotplug in and out the memory at run-time

hotplug code does that.


> 5. Exchange pointers between coherent device memory and normal memory
> for the compute on the coherent device memory to use

I dont see anything preventing that from occurring right now. Thats a
device issue with doing proper virtual to physical mapping right?

> I could list further things, but largely coherent device memory is like
> system memory except that we believe that things like auto-numa balancing
> and kswapd will not work well due to lack of information about references
> and faults.

Ok so far I do not see that we need coherent nodes at all.

> Some of the mm-summit notes are at https://lwn.net/Articles/717601/
> The goals align with HMM, except that the device memory is coherent. HMM
> has a CDM variation as well.

I was at the presentation but at that point you were interested in a
different approach it seems.

> We've been using the term coherent device memory (CDM). I could rephrase the
> text and documentation for consistency. Would you prefer a different term?

Hotplug memory node?

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