Re: [RFC] memory tiering: use small chunk size and more tiers

[Huang, Ying]

Michal Hocko <mhocko@xxxxxxxx> writes:

> On Wed 02-11-22 08:39:49, Huang, Ying wrote:
>> Michal Hocko <mhocko@xxxxxxxx> writes:
>> 
>> > On Mon 31-10-22 09:33:49, Huang, Ying wrote:
>> > [...]
>> >> In the upstream implementation, 4 tiers are possible below DRAM.  That's
>> >> enough for now.  But in the long run, it may be better to define more.
>> >> 100 possible tiers below DRAM may be too extreme.
>> >
>> > I am just curious. Is any configurations with more than couple of tiers
>> > even manageable? I mean applications have been struggling even with
>> > regular NUMA systems for years and vast majority of them is largerly
>> > NUMA unaware. How are they going to configure for a more complex system
>> > when a) there is no resource access control so whatever you aim for
>> > might not be available and b) in which situations there is going to be a
>> > demand only for subset of tears (GPU memory?) ?
>> 
>> Sorry for confusing.  I think that there are only several (less than 10)
>> tiers in a system in practice.  Yes, here, I suggested to define 100 (10
>> in the later text) POSSIBLE tiers below DRAM.  My intention isn't to
>> manage a system with tens memory tiers.  Instead, my intention is to
>> avoid to put 2 memory types into one memory tier by accident via make
>> the abstract distance range of each memory tier as small as possible.
>> More possible memory tiers, smaller abstract distance range of each
>> memory tier.
>
> TBH I do not really understand how tweaking ranges helps anything.
> IIUC drivers are free to assign any abstract distance so they will clash
> without any higher level coordination.

Yes.  That's possible.  Each memory tier corresponds to one abstract
distance range.  The larger the range is, the higher the possibility of
clashing is.  So I suggest to make the abstract distance range smaller
to reduce the possibility of clashing.

Best Regards,
Huang, Ying