Re: [PATCH v2 4/5] mm: FLEXIBLE_THP for improved performance

["Huang, Ying" <ying.huang@intel.com>]

Ryan Roberts <ryan.roberts@arm.com> writes:

> On 10/07/2023 10:18, Huang, Ying wrote:
>> Ryan Roberts <ryan.roberts@arm.com> writes:
>> 
>>> On 10/07/2023 04:03, Huang, Ying wrote:
>>>> Ryan Roberts <ryan.roberts@arm.com> writes:
>>>>
>>>>> On 07/07/2023 15:07, David Hildenbrand wrote:
>>>>>> On 07.07.23 15:57, Matthew Wilcox wrote:
>>>>>>> On Fri, Jul 07, 2023 at 01:29:02PM +0200, David Hildenbrand wrote:
>>>>>>>> On 07.07.23 11:52, Ryan Roberts wrote:
>>>>>>>>> On 07/07/2023 09:01, Huang, Ying wrote:
>>>>>>>>>> Although we can use smaller page order for FLEXIBLE_THP, it's hard to
>>>>>>>>>> avoid internal fragmentation completely.  So, I think that finally we
>>>>>>>>>> will need to provide a mechanism for the users to opt out, e.g.,
>>>>>>>>>> something like "always madvise never" via
>>>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled.  I'm not sure whether it's
>>>>>>>>>> a good idea to reuse the existing interface of THP.
>>>>>>>>>
>>>>>>>>> I wouldn't want to tie this to the existing interface, simply because that
>>>>>>>>> implies that we would want to follow the "always" and "madvise" advice too;
>>>>>>>>> That
>>>>>>>>> means that on a thp=madvise system (which is certainly the case for android and
>>>>>>>>> other client systems) we would have to disable large anon folios for VMAs that
>>>>>>>>> haven't explicitly opted in. That breaks the intention that this should be an
>>>>>>>>> invisible performance boost. I think it's important to set the policy for
>>>>>>>>> use of
>>>>>>>>
>>>>>>>> It will never ever be a completely invisible performance boost, just like
>>>>>>>> ordinary THP.
>>>>>>>>
>>>>>>>> Using the exact same existing toggle is the right thing to do. If someone
>>>>>>>> specify "never" or "madvise", then do exactly that.
>>>>>>>>
>>>>>>>> It might make sense to have more modes or additional toggles, but
>>>>>>>> "madvise=never" means no memory waste.
>>>>>>>
>>>>>>> I hate the existing mechanisms.  They are an abdication of our
>>>>>>> responsibility, and an attempt to blame the user (be it the sysadmin
>>>>>>> or the programmer) of our code for using it wrongly.  We should not
>>>>>>> replicate this mistake.
>>>>>>
>>>>>> I don't agree regarding the programmer responsibility. In some cases the
>>>>>> programmer really doesn't want to get more memory populated than requested --
>>>>>> and knows exactly why setting MADV_NOHUGEPAGE is the right thing to do.
>>>>>>
>>>>>> Regarding the madvise=never/madvise/always (sys admin decision), memory waste
>>>>>> (and nailing down bugs or working around them in customer setups) have been very
>>>>>> good reasons to let the admin have a word.
>>>>>>
>>>>>>>
>>>>>>> Our code should be auto-tuning.  I posted a long, detailed outline here:
>>>>>>> https://lore.kernel.org/linux-mm/Y%2FU8bQd15aUO97vS@casper.infradead.org/
>>>>>>>
>>>>>>
>>>>>> Well, "auto-tuning" also should be perfect for everybody, but once reality
>>>>>> strikes you know it isn't.
>>>>>>
>>>>>> If people don't feel like using THP, let them have a word. The "madvise" config
>>>>>> option is probably more controversial. But the "always vs. never" absolutely
>>>>>> makes sense to me.
>>>>>>
>>>>>>>> I remember I raised it already in the past, but you *absolutely* have to
>>>>>>>> respect the MADV_NOHUGEPAGE flag. There is user space out there (for
>>>>>>>> example, userfaultfd) that doesn't want the kernel to populate any
>>>>>>>> additional page tables. So if you have to respect that already, then also
>>>>>>>> respect MADV_HUGEPAGE, simple.
>>>>>>>
>>>>>>> Possibly having uffd enabled on a VMA should disable using large folios,
>>>>>>
>>>>>> There are cases where we enable uffd *after* already touching memory (postcopy
>>>>>> live migration in QEMU being the famous example). That doesn't fly.
>>>>>>
>>>>>>> I can get behind that.  But the notion that userspace knows what it's
>>>>>>> doing ... hahaha.  Just ignore the madvise flags.  Userspace doesn't
>>>>>>> know what it's doing.
>>>>>>
>>>>>> If user space sets MADV_NOHUGEPAGE, it exactly knows what it is doing ... in
>>>>>> some cases. And these include cases I care about messing with sparse VM memory :)
>>>>>>
>>>>>> I have strong opinions against populating more than required when user space set
>>>>>> MADV_NOHUGEPAGE.
>>>>>
>>>>> I can see your point about honouring MADV_NOHUGEPAGE, so think that it is
>>>>> reasonable to fallback to allocating an order-0 page in a VMA that has it set.
>>>>> The app has gone out of its way to explicitly set it, after all.
>>>>>
>>>>> I think the correct behaviour for the global thp controls (cmdline and sysfs)
>>>>> are less obvious though. I could get on board with disabling large anon folios
>>>>> globally when thp="never". But for other situations, I would prefer to keep
>>>>> large anon folios enabled (treat "madvise" as "always"),
>>>>
>>>> If we have some mechanism to auto-tune the large folios usage, for
>>>> example, detect the internal fragmentation and split the large folio,
>>>> then we can use thp="always" as default configuration.  If my memory
>>>> were correct, this is what Johannes and Alexander is working on.
>>>
>>> Could you point me to that work? I'd like to understand what the mechanism is.
>>> The other half of my work aims to use arm64's pte "contiguous bit" to tell the
>>> HW that a span of PTEs share the same mapping and is therefore coalesced into a
>>> single TLB entry. The side effect of this, however, is that we only have a
>>> single access and dirty bit for the whole contpte extent. So I'd like to avoid
>>> any mechanism that relies on getting access/dirty at the base page granularity
>>> for a large folio.
>> 
>> Please take a look at the THP shrinker patchset,
>> 
>> https://lore.kernel.org/linux-mm/cover.1667454613.git.alexlzhu@fb.com/
>
> Thanks!
>
>> 
>>>>
>>>>> with the argument that
>>>>> their order is much smaller than traditional THP and therefore the internal
>>>>> fragmentation is significantly reduced.
>>>>
>>>> Do you have any data for this?
>>>
>>> Some; its partly based on intuition that the smaller the allocation unit, the
>>> smaller the internal fragmentation. And partly on peak memory usage data I've
>>> collected for the benchmarks I'm running, comparing baseline-4k kernel with
>>> baseline-16k and baseline-64 kernels along with a 4k kernel that supports large
>>> anon folios (I appreciate that's not exactly what we are talking about here, and
>>> it's not exactly an extensive set of results!):
>>>
>>>
>>> Kernel Compliation with 8 Jobs:
>>> | kernel        |   peak |
>>> |:--------------|-------:|
>>> | baseline-4k   |   0.0% |
>>> | anonfolio     |   0.1% |
>>> | baseline-16k  |   6.3% |
>>> | baseline-64k  |  28.1% |
>>>
>>>
>>> Kernel Compliation with 80 Jobs:
>>> | kernel        |   peak |
>>> |:--------------|-------:|
>>> | baseline-4k   |   0.0% |
>>> | anonfolio     |   1.7% |
>>> | baseline-16k  |   2.6% |
>>> | baseline-64k  |  12.3% |
>>>
>> 
>> Why is anonfolio better than baseline-64k if you always allocate 64k
>> anonymous folio?  Because page cache uses 64k in baseline-64k?
>
> No, because the VMA boundaries are aligned to 4K and not 64K. Large Anon Folios
> only allocates a 64K folio if it does not breach the bounds of the VMA (and if
> it doesn't overlap other allocated PTEs).

Thanks for explanation!

We will use more memory for file cache too for baseline-64k, right?  So,
you observed much more anonymous pages, but not so for file cache pages?

>> 
>> We may need to test some workloads with sparse access patterns too.
>
> Yes, I agree if you have a workload with a pathalogical memory access pattern
> where it writes to addresses with a stride of 64K, all contained in a single
> VMA, then you will end up allocating 16x the memory. This is obviously an
> unrealistic extreme though.

I think that there should be some realistic workload which has sparse
access patterns.

Best Regards,
Huang, Ying

>> 
>>>>
>>>>> I really don't want to end up with user
>>>>> space ever having to opt-in (with MADV_HUGEPAGE) to see the benefits of large
>>>>> anon folios.
>>>>>
>>>>> I still feel that it would be better for the thp and large anon folio controls
>>>>> to be independent though - what's the argument for tying them together?
>>>>>
>>