Re: [RFC PATCH 0/5] Memory access profiler(IBS) driven NUMA balancing

[Huang, Ying]

Bharata B Rao <bharata@xxxxxxx> writes:

> On 2/13/2023 8:26 AM, Huang, Ying wrote:
>> Bharata B Rao <bharata@xxxxxxx> writes:
>> 
>>> On 2/8/2023 11:33 PM, Peter Zijlstra wrote:
>>>> On Wed, Feb 08, 2023 at 01:05:28PM +0530, Bharata B Rao wrote:
>>>>
>>>>
>>>>> - Hardware provided access information could be very useful for driving
>>>>>   hot page promotion in tiered memory systems. Need to check if this
>>>>>   requires different tuning/heuristics apart from what NUMA balancing
>>>>>   already does.
>>>>
>>>> I think Huang Ying looked at that from the Intel POV and I think the
>>>> conclusion was that it doesn't really work out. What you need is
>>>> frequency information, but the PMU doesn't really give you that. You
>>>> need to process a *ton* of PMU data in-kernel.
>>>
>>> What I am doing here is to feed the access data into NUMA balancing which
>>> already has the logic to aggregate that at task and numa group level and
>>> decide if that access is actionable in terms of migrating the page. In this
>>> context, I am not sure about the frequency information that you and Dave
>>> are mentioning. AFAIU, existing NUMA balancing takes care of taking
>>> action, IBS becomes an alternative source of access information to NUMA
>>> hint faults.
>> 
>> We do need frequency information to determine whether a page is hot
>> enough to be migrated to the fast memory (promotion).  What PMU provided
>> is just "recently" accessed pages, not "frequently" accessed pages.  For
>> current NUMA balancing implementation, please check
>> NUMA_BALANCING_MEMORY_TIERING in should_numa_migrate_memory().  In
>> general, it estimates the page access frequency via measuring the
>> latency between page table scanning and page fault, the shorter the
>> latency, the higher the frequency.  This isn't perfect, but provides a
>> starting point.  You need to consider how to get frequency information
>> via PMU.  For example, you may count access number for each page, aging
>> them periodically, and get hot threshold via some statistics.
>
> For the tiered memory hot page promotion case of NUMA balancing, we will
> have to maintain frequency information in software when such information
> isn't available from the hardware.

Yes.  It's challenging to calculate frequency information.  Please
consider how to do that.

Best Regards,
Huang, Ying