Re: RFC: Memory Tiering Kernel Interfaces (v2)

[Wei Xu <weixugc@google.com>]

On Wed, May 25, 2022 at 4:48 AM Jonathan Cameron
<Jonathan.Cameron@huawei.com> wrote:
>
> On Wed, 25 May 2022 17:47:33 +1000
> Alistair Popple <apopple@nvidia.com> wrote:
>
> > Wei Xu <weixugc@google.com> writes:
> >
> > > On Tue, May 24, 2022 at 6:27 AM Aneesh Kumar K.V
> > > <aneesh.kumar@linux.ibm.com> wrote:
> > >>
> > >> Wei Xu <weixugc@google.com> writes:
> > >>
> > >> > On Wed, May 18, 2022 at 5:00 AM Jonathan Cameron
> > >> > <Jonathan.Cameron@huawei.com> wrote:
> > >> >>
> > >> >> On Wed, 18 May 2022 00:09:48 -0700
> > >> >> Wei Xu <weixugc@google.com> wrote:
> > >>
> > >> ...
> > >>
> > >> > Nice :)
> > >> >>
> > >> >> Initially I thought this was over complicated when compared to just leaving space, but
> > >> >> after a chat with Hesham just now you have us both convinced that this is an elegant solution.
> > >> >>
> > >> >> Few corners probably need fleshing out:
> > >> >> *  Use of an allocator for new tiers. Flat number at startup, or new one on write of unique
> > >> >>    value to set_memtier perhaps?  Also whether to allow drivers to allocate (I think
> > >> >>    we should).
> > >> >> *  Multiple tiers with same rank.  My assumption is from demotion path point of view you
> > >> >>    fuse them (treat them as if they were a single tier), but keep them expressed
> > >> >>    separately in the sysfs interface so that the rank can be changed independently.
> > >> >> *  Some guidance on what values make sense for given rank default that might be set by
> > >> >>    a driver. If we have multiple GPU vendors, and someone mixes them in a system we
> > >> >>    probably don't want the default values they use to result in demotion between them.
> > >> >>    This might well be a guidance DOC or appropriate set of #define
> > >> >
> > >> > All of these are good ideas, though I am afraid that these can make
> > >> > tier management too complex for what it's worth.
> > >> >
> > >> > How about an alternative tier numbering scheme that uses major.minor
> > >> > device IDs?  For simplicity, we can just start with 3 major tiers.
> > >> > New tiers can be inserted in-between using minor tier IDs.
> > >>
> > >>
> > >> What drives the creation of a new memory tier here?  Jonathan was
> > >> suggesting we could do something similar to writing to set_memtier for
> > >> creating a new memory tier.
> > >>
> > >> $ echo "memtier128" > sys/devices/system/node/node1/set_memtier
> > >>
> > >> But I am wondering whether we should implement that now. If we keep
> > >> "rank" concept and detach tier index (memtier0 is the memory tier with
> > >> index 0) separate from rank, I assume we have enough flexibility for a
> > >> future extension that will allow us to create a memory tier from userspace
> > >> and assigning it a rank value that helps the device to be placed before or
> > >> after DRAM in demotion order.
> > >>
> > >> ie, For now we will only have memtier0, memtier1, memtier2. We won't add
> > >> dynamic creation of memory tiers and the above memory tiers will have
> > >> rank value 0, 1, 2 according with demotion order 0 -> 1 -> 2.
> > >
> > > Great. So the consensus is to go with the "rank" approach.  The above
> > > sounds good to me as a starting point.
> >
> > The rank approach seems good to me too.
>
> Rank is good, but I do slightly worry about accidentally defining ABI
> that people care about with the particular numbers used for the initial ranks.
>
> Maybe just x100 on all of them to allow things in between with no change to
> this initial set of 3?  So 0, 100, 200

I strongly support this, which is also my original intention for rank
values. I'd suggest to even remove 0 to avoid it becoming a special
value that userspace depends on.

> Jonathan
>
> >
> >  - Alistair
> >
> > >> -aneesh
>