Re: [RFC 0/6] mm: improve page allocator scalability via splitting zones

David Hildenbrand
2023-05-16T10:30:30Z

On 16.05.23 11:38, Huang, Ying wrote:
> Michal Hocko <mhocko@suse.com> writes:
>
>> On Fri 12-05-23 10:55:21, Huang, Ying wrote:
>>> Hi, Michal,
>>>
>>> Thanks for comments!
>>>
>>> Michal Hocko <mhocko@suse.com> writes:
>>>
>>>> On Thu 11-05-23 14:56:01, Huang Ying wrote:
>>>>> The patchset is based on upstream v6.3.
>>>>>
>>>>> More and more cores are put in one physical CPU (usually one NUMA node
>>>>> too). In 2023, one high-end server CPU has 56, 64, or more cores.
>>>>> Even more cores per physical CPU are planned for future CPUs. While
>>>>> all cores in one physical CPU will contend for the page allocation on
>>>>> one zone in most cases. This causes heavy zone lock contention in
>>>>> some workloads. And the situation will become worse and worse in the
>>>>> future.
>>>>>
>>>>> For example, on an 2-socket Intel server machine with 224 logical
>>>>> CPUs, if the kernel is built with `make -j224`, the zone lock
>>>>> contention cycles% can reach up to about 12.7%.
>>>>>
>>>>> To improve the scalability of the page allocation, in this series, we
>>>>> will create one zone instance for each about 256 GB memory of a zone
>>>>> type generally. That is, one large zone type will be split into
>>>>> multiple zone instances. Then, different logical CPUs will prefer
>>>>> different zone instances based on the logical CPU No. So the total
>>>>> number of logical CPUs contend on one zone will be reduced. Thus the
>>>>> scalability is improved.
>>>>
>>>> It is not really clear to me why you need a new zone for all this rather
>>>> than partition free lists internally within the zone? Essentially to
>>>> increase the current two level system to 3: per cpu caches, per cpu
>>>> arenas and global fallback.
>>>
>>> Sorry, I didn't get your idea here. What is per cpu arenas? What's the
>>> difference between it and per cpu caches (PCP)?
>>
>> Sorry, I didn't give this much thought than the above. Essentially, we
>> have 2 level system right now. Pcp caches should reduce the contention
>> on the per cpu level and that should work reasonably well, if you manage
>> to align batch sizes to the workload AFAIK. If this is not sufficient
>> then why to add the full zone rather than to add another level that
>> caches across a larger than a cpu unit. Maybe a core?
>>
>> This might be a wrong way around going for this but there is not much
>> performance analysis about the source of the lock contention so I am
>> mostly guessing.
>
> I guess that the page allocation scalability will be improved if we put
> more pages in the per CPU caches, or add another level of cache for
> multiple logical CPUs. Because more page allocation requirements can be
> satisfied without acquiring zone lock.
>
> As other caching system, there are always cases that the caches are
> drained and too many requirements goes to underlying slow layer (zone
> here). For example, if a workload needs to allocate a huge number of
> pages (larger than cache size) in parallel, it will run into zone lock
> contention finally. The situation will became worse and worse if we
> share one zone with more and more logical CPUs. Which is the trend in
> industry now. Per my understanding, we can observe the high zone lock
> contention cycles in kbuild test because of that.
>
> So, per my understanding, to improve the page allocation scalability in
> bad situations (that is, caching doesn't work well enough), we need to
> restrict the number of logical CPUs that share one zone. This series is
> an attempt for that. Better caching can increase the good situations
> and reduce the bad situations. But it seems hard to eliminate all bad
> situations.
>
> From another perspective, we don't install more and more memory for each
> logical CPU. This makes it hard to enlarge the default per-CPU cache
> size.
>
>>>> I am also missing some information why pcp caches tunning is not
>>>> sufficient.
>>>
>>> PCP does improve the page allocation scalability greatly! But it
>>> doesn't help much for workloads that allocating pages on one CPU and
>>> free them in different CPUs. PCP tuning can improve the page allocation
>>> scalability for a workload greatly. But it's not trivial to find the
>>> best tuning parameters for various workloads and workload run time
>>> statuses (workloads may have different loads and memory requirements at
>>> different time). And we may run different workloads on different
>>> logical CPUs of the system. This also makes it hard to find the best
>>> PCP tuning globally.
>>
>> Yes this makes sense. Does that mean that the global pcp tuning is not
>> keeping up and we need to be able to do more auto-tuning on local bases
>> rather than global?
>
> Similar as above, I think that PCP helps the good situations performance
> greatly, and splitting zone can help the bad situations scalability.
> They are working at the different levels.
>
> As for PCP auto-tuning, I think that it's hard to implement it to
> resolve all problems (that is, makes PCP never be drained).
>
> And auto-tuning doesn't sound easy. Do you have some idea of how to do
> that?

If we could avoid instantiating more zones and rather improve existing
mechanisms (PCP), that would be much more preferred IMHO. I'm sure it's
not easy, but that shouldn't stop us from trying ;)

I did not look into the details of this proposal, but seeing the change
in include/linux/page-flags-layout.h scares me. Further, I'm not so sure
how that change really interacts with hot(un)plug of memory ... on a
quick glimpse I feel like this series hacks the code such that such that
the split works based on the boot memory size ...

I agree with Michal that looking into auto-tuning PCP would be
preferred. If that can't be done, adding another layer might end up
cleaner and eventually cover more use cases.

[I recall there was once a proposal to add a 3rd layer to limit
fragmenation to individual memory blocks; but the granularity was rather
small and there were also some concerns that I don't recall anymore]

--
Thanks,

David / dhildenb