Re: [RFC PATCH 00/26] mm: reliable huge page allocator

Vlastimil Babka
2023-04-19T10:57:11Z

On 4/19/23 04:08, Johannes Weiner wrote:
> Hi Kirill, thanks for taking a look so quickly.
>
> On Wed, Apr 19, 2023 at 02:54:02AM +0300, Kirill A. Shutemov wrote:
>> On Tue, Apr 18, 2023 at 03:12:47PM -0400, Johannes Weiner wrote:
>> > This series proposes to make THP allocations reliable by enforcing
>> > pageblock hygiene, and aligning the allocator, reclaim and compaction
>> > on the pageblock as the base unit for managing free memory. All orders
>> > up to and including the pageblock are made first-class requests that
>> > (outside of OOM situations) are expected to succeed without
>> > exceptional investment by the allocating thread.
>> >
>> > A neutral pageblock type is introduced, MIGRATE_FREE. The first
>> > allocation to be placed into such a block claims it exclusively for
>> > the allocation's migratetype. Fallbacks from a different type are no
>> > longer allowed, and the block is "kept open" for more allocations of
>> > the same type to ensure tight grouping. A pageblock becomes neutral
>> > again only once all its pages have been freed.
>>
>> Sounds like this will cause earlier OOM, no?
>>
>> I guess with 2M pageblock on 64G server it shouldn't matter much. But how
>> about smaller machines?
>
> Yes, it's a tradeoff.
>
> It's not really possible to reduce external fragmentation and increase
> contiguity, without also increasing the risk of internal fragmentation
> to some extent. The tradeoff is slighly less but overall faster memory.
>
> A 2M block size *seems* reasonable for most current setups. It's
> actually still somewhat on the lower side, if you consider that we had
> 4k blocks when memory was a few megabytes. (4k pages for 4M RAM is the
> same ratio as 2M pages for 2G RAM. My phone has 8G and my desktop 32G.
> 64G is unusually small for a datacenter server.)
>
> I wouldn't be opposed to sticking this behind a separate config option
> if there are setups that WOULD want to keep the current best-effort
> compaction without the block hygiene. But obviously, from a
> maintenance POV life would be much easier if we didn't have to.

As much as tunables are frowned upon in general, this could make sense to me
even as a runtime tunable (maybe with defaults based on how large the
system is), because a datacenter server and a phone is after all not the
same thing. But of course it would be preferrable to find out it works
reasonably well even for the smaller systems. For example we already do
completely disable mobility grouping if there's too little RAM for it to
make sense, which is somewhat similar (but not completely identical) decision.

> FWIF, I have been doing tests in an environment constrained to 2G and
> haven't had any issues with premature OOMs. But I'm happy to test
> other situations and workloads that might be of interest to people.
>
>> > Reclaim and compaction are changed from partial block reclaim to
>> > producing whole neutral page blocks.
>>
>> How does it affect allocation latencies? I see direct compact stall grew
>> substantially. Hm?
>
> Good question.
>
> There are 260 more compact stalls but also 1,734 more successful THP
> allocations. And 1,433 fewer allocation stalls. There seems to be much
> less direct work performed per successful allocation.

Yeah if there's a workload that uses THP madvise to indicate it prefers the
compaction stalls to base page fallbacks, and compaction is more sucessful,
it won't defer further attempts so as a result there will be more stalls.
What we should watch out for are rather latencies of allocations that don't
prefer the stalls, but might now be forced to clean up new MIGRATE_FREE
pageblocks for their order-0 allocation that would previously just fallback,
etc.

> But of course, that's not the whole story. Let me trace the actual
> latencies.
>
> Thanks for your thoughts!
> Johannes