Re: [PATCH RESEND 1/4] memory tiering: add abstract distance calculation algorithms management

[Huang, Ying]

Alistair Popple <apopple@xxxxxxxxxx> writes:

> "Huang, Ying" <ying.huang@xxxxxxxxx> writes:
>
>> Alistair Popple <apopple@xxxxxxxxxx> writes:
>>
>>> "Huang, Ying" <ying.huang@xxxxxxxxx> writes:
>>>
>>>>>> And, I don't think that we are forced to use the general notifier
>>>>>> chain interface in all memory device drivers.  If the memory device
>>>>>> driver has better understanding of the memory device, it can use other
>>>>>> way to determine abstract distance.  For example, a CXL memory device
>>>>>> driver can identify abstract distance by itself.  While other memory
>>>>>> device drivers can use the general notifier chain interface at the
>>>>>> same time.
>>>>>
>>>>> Whilst I think personally I would find that flexibility useful I am
>>>>> concerned it means every driver will just end up divining it's own
>>>>> distance rather than ensuring data in HMAT/CDAT/etc. is correct. That
>>>>> would kind of defeat the purpose of it all then.
>>>>
>>>> But we have no way to enforce that too.
>>>
>>> Enforce that HMAT/CDAT/etc. is correct? Agree we can't enforce it, but
>>> we can influence it. If drivers can easily ignore the notifier chain and
>>> do their own thing that's what will happen.
>>
>> IMHO, both enforce HMAT/CDAT/etc is correct and enforce drivers to use
>> general interface we provided.  Anyway, we should try to make HMAT/CDAT
>> works well, so drivers want to use them :-)
>
> Exactly :-)
>
>>>>>> While other memory device drivers can use the general notifier chain
>>>>>> interface at the same time.
>>>
>>> How would that work in practice though? The abstract distance as far as
>>> I can tell doesn't have any meaning other than establishing preferences
>>> for memory demotion order. Therefore all calculations are relative to
>>> the rest of the calculations on the system. So if a driver does it's own
>>> thing how does it choose a sensible distance? IHMO the value here is in
>>> coordinating all that through a standard interface, whether that is HMAT
>>> or something else.
>>
>> Only if different algorithms follow the same basic principle.  For
>> example, the abstract distance of default DRAM nodes are fixed
>> (MEMTIER_ADISTANCE_DRAM).  The abstract distance of the memory device is
>> in linear direct proportion to the memory latency and inversely
>> proportional to the memory bandwidth.  Use the memory latency and
>> bandwidth of default DRAM nodes as base.
>>
>> HMAT and CDAT report the raw memory latency and bandwidth.  If there are
>> some other methods to report the raw memory latency and bandwidth, we
>> can use them too.
>
> Argh! So we could address my concerns by having drivers feed
> latency/bandwidth numbers into a standard calculation algorithm right?
> Ie. Rather than having drivers calculate abstract distance themselves we
> have the notifier chains return the raw performance data from which the
> abstract distance is derived.

Now, memory device drivers only need a general interface to get the
abstract distance from the NUMA node ID.  In the future, if they need
more interfaces, we can add them.  For example, the interface you
suggested above.

--
Best Regards,
Huang, Ying