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:
>
>> Hi, Alistair,
>>
>> Sorry for late response.  Just come back from vacation.
>
> Ditto for this response :-)
>
> I see Andrew has taken this into mm-unstable though, so my bad for not
> getting around to following all this up sooner.
>
>> Alistair Popple <apopple@xxxxxxxxxx> writes:
>>
>>> "Huang, Ying" <ying.huang@xxxxxxxxx> writes:
>>>
>>>> Alistair Popple <apopple@xxxxxxxxxx> writes:
>>>>
>>>>> "Huang, Ying" <ying.huang@xxxxxxxxx> writes:
>>>>>
>>>>>> Alistair Popple <apopple@xxxxxxxxxx> writes:
>>>>>>
>>>>>>>>>> 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.
>>>
>>> Huh? Memory device drivers (ie. dax/kmem.c) don't care about abstract
>>> distance, it's a meaningless number. The only reason they care about it
>>> is so they can pass it to alloc_memory_type():
>>>
>>> struct memory_dev_type *alloc_memory_type(int adistance)
>>>
>>> Instead alloc_memory_type() should be taking bandwidth/latency numbers
>>> and the calculation of abstract distance should be done there. That
>>> resovles the issues about how drivers are supposed to devine adistance
>>> and also means that when CDAT is added we don't have to duplicate the
>>> calculation code.
>>
>> In the current design, the abstract distance is the key concept of
>> memory types and memory tiers.  And it is used as interface to allocate
>> memory types.  This provides more flexibility than some other interfaces
>> (e.g. read/write bandwidth/latency).  For example, in current
>> dax/kmem.c, if HMAT isn't available in the system, the default abstract
>> distance: MEMTIER_DEFAULT_DAX_ADISTANCE is used.  This is still useful
>> to support some systems now.  On a system without HMAT/CDAT, it's
>> possible to calculate abstract distance from ACPI SLIT, although this is
>> quite limited.  I'm not sure whether all systems will provide read/write
>> bandwith/latency data for all memory devices.
>>
>> HMAT and CDAT or some other mechanisms may provide the read/write
>> bandwidth/latency data to be used to calculate abstract distance.  For
>> them, we can provide a shared implementation in mm/memory-tiers.c to map
>> from read/write bandwith/latency to the abstract distance.  Can this
>> solve your concerns about the consistency among algorithms?  If so, we
>> can do that when we add the second algorithm that needs that.
>
> I guess it would address my concerns if we did that now. I don't see why
> we need to wait for a second implementation for that though - the whole
> series seems to be built around adding a framework for supporting
> multiple algorithms even though only one exists. So I think we should
> support that fully, or simplfy the whole thing and just assume the only
> thing that exists is HMAT and get rid of the general interface until a
> second algorithm comes along.

We will need a general interface even for one algorithm implementation.
Because it's not good to make a dax subsystem driver (dax/kmem) to
depend on a ACPI subsystem driver (acpi/hmat).  We need some general
interface at subsystem level (memory tier here) between them.

Best Regards,
Huang, Ying