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 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/

>> 
>>> 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?

We may need to test some workloads with sparse access patterns too.

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?
>>>