Re: [PATCH v5 3/5] mm: LARGE_ANON_FOLIO for improved performance

[Yin, Fengwei]

On 8/31/2023 3:57 PM, David Hildenbrand wrote:
> On 31.08.23 03:40, Huang, Ying wrote:
>> Ryan Roberts <ryan.roberts@xxxxxxx> writes:
>>
>>> On 15/08/2023 22:32, Huang, Ying wrote:
>>>> Hi, Ryan,
>>>>
>>>> Ryan Roberts <ryan.roberts@xxxxxxx> writes:
>>>>
>>>>> Introduce LARGE_ANON_FOLIO feature, which allows anonymous memory to be
>>>>> allocated in large folios of a determined order. All pages of the large
>>>>> folio are pte-mapped during the same page fault, significantly reducing
>>>>> the number of page faults. The number of per-page operations (e.g. ref
>>>>> counting, rmap management lru list management) are also significantly
>>>>> reduced since those ops now become per-folio.
>>>>>
>>>>> The new behaviour is hidden behind the new LARGE_ANON_FOLIO Kconfig,
>>>>> which defaults to disabled for now; The long term aim is for this to
>>>>> defaut to enabled, but there are some risks around internal
>>>>> fragmentation that need to be better understood first.
>>>>>
>>>>> Large anonymous folio (LAF) allocation is integrated with the existing
>>>>> (PMD-order) THP and single (S) page allocation according to this policy,
>>>>> where fallback (>) is performed for various reasons, such as the
>>>>> proposed folio order not fitting within the bounds of the VMA, etc:
>>>>>
>>>>>                  | prctl=dis | prctl=ena   | prctl=ena     | prctl=ena
>>>>>                  | sysfs=X   | sysfs=never | sysfs=madvise | sysfs=always
>>>>> ----------------|-----------|-------------|---------------|-------------
>>>>> no hint         | S         | LAF>S       | LAF>S         | THP>LAF>S
>>>>> MADV_HUGEPAGE   | S         | LAF>S       | THP>LAF>S     | THP>LAF>S
>>>>> MADV_NOHUGEPAGE | S         | S           | S             | S
>>>>
>>>> IMHO, we should use the following semantics as you have suggested
>>>> before.
>>>>
>>>>                  | prctl=dis | prctl=ena   | prctl=ena     | prctl=ena
>>>>                  | sysfs=X   | sysfs=never | sysfs=madvise | sysfs=always
>>>> ----------------|-----------|-------------|---------------|-------------
>>>> no hint         | S         | S           | LAF>S         | THP>LAF>S
>>>> MADV_HUGEPAGE   | S         | S           | THP>LAF>S     | THP>LAF>S
>>>> MADV_NOHUGEPAGE | S         | S           | S             | S
>>>>
>>>> Or even,
>>>>
>>>>                  | prctl=dis | prctl=ena   | prctl=ena     | prctl=ena
>>>>                  | sysfs=X   | sysfs=never | sysfs=madvise | sysfs=always
>>>> ----------------|-----------|-------------|---------------|-------------
>>>> no hint         | S         | S           | S             | THP>LAF>S
>>>> MADV_HUGEPAGE   | S         | S           | THP>LAF>S     | THP>LAF>S
>>>> MADV_NOHUGEPAGE | S         | S           | S             | S
>>>>
>>>>  From the implementation point of view, PTE mapped PMD-sized THP has
>>>> almost no difference with LAF (just some small sized THP).  It will be
>>>> confusing to distinguish them from the interface point of view.
>>>>
>>>> So, IMHO, the real difference is the policy.  For example, prefer
>>>> PMD-sized THP, prefer small sized THP, or fully auto.  The sysfs
>>>> interface is used to specify system global policy.  In the long term, it
>>>> can be something like below,
>>>>
>>>> never:      S               # disable all THP
>>>> madvise:                    # never by default, control via madvise()
>>>> always:     THP>LAF>S       # prefer PMD-sized THP in fact
>>>> small:      LAF>S           # prefer small sized THP
>>>> auto:                       # use in-kernel heuristics for THP size
>>>>
>>>> But it may be not ready to add new policies now.  So, before the new
>>>> policies are ready, we can add a debugfs interface to override the
>>>> original policy in /sys/kernel/mm/transparent_hugepage/enabled.  After
>>>> we have tuned enough workloads, collected enough data, we can add new
>>>> policies to the sysfs interface.
>>>
>>> I think we can all imagine many policy options. But we don't really have much
>>> evidence yet for what it best. The policy I'm currently using is intended to
>>> give some flexibility for testing (use LAF without THP by setting sysfs=never,
>>> use THP without LAF by compiling without LAF) without adding any new knobs at
>>> all. Given that, surely we can defer these decisions until we have more data?
>>>
>>> In the absence of data, your proposed solution sounds very sensible to me. But
>>> for the purposes of scaling up perf testing, I don't think its essential given
>>> the current policy will also produce the same options.
>>>
>>> If we were going to add a debugfs knob, I think the higher priority would be a
>>> knob to specify the folio order. (but again, I would rather avoid if possible).
>>
>> I totally understand we need some way to control PMD-sized THP and LAF
>> to tune the workload, and nobody likes debugfs knob.
>>
>> My concern about interface is that we have no way to disable LAF
>> system-wise without rebuilding the kernel.  In the future, should we add
>> a new policy to /sys/kernel/mm/transparent_hugepage/enabled to be
>> stricter than "never"?  "really_never"?
> 
> Let's talk about that in a bi-weekly MM session. (I proposed it as a topic for next week).

The time slot of the meeting is not friendly to our timezone. Like
it's 1 or 2 AM. Yes. I know it's very hard to find a good time slot
for US, EU and Asia. :(.

So maybe we still need to discuss it through mail?


Regards
Yin, Fengwei

> 
> As raised in another mail, we can then discuss
> * how we want to call this feature (transparent large pages? there is
>   the concern that "THP" might confuse users. Maybe we can consider
>   "large" the more generic version and "huge" only PMD-size, TBD)
> * how to expose it in stats towards the user (e.g., /proc/meminfo)
> * which minimal toggles we want
> 
> I think there *really* has to be a way to disable it for a running system, otherwise no distro will dare pulling it in, even after we figured out the other stuff.
> 
> Note that for the pagecache, large folios can be disabled and distributions are actively making use of that.
>