Re: [PATCH v4 0/6] Swap-out mTHP without splitting

Ryan Roberts
2024-03-12T13:57:03Z

On 12/03/2024 08:49, Ryan Roberts wrote:
> On 12/03/2024 08:01, Huang, Ying wrote:
>> Ryan Roberts <ryan.roberts@arm.com> writes:
>>
>>> Hi All,
>>>
>>> This series adds support for swapping out multi-size THP (mTHP) without needing
>>> to first split the large folio via split_huge_page_to_list_to_order(). It
>>> closely follows the approach already used to swap-out PMD-sized THP.
>>>
>>> There are a couple of reasons for swapping out mTHP without splitting:
>>>
>>> - Performance: It is expensive to split a large folio and under extreme memory
>>> pressure some workloads regressed performance when using 64K mTHP vs 4K
>>> small folios because of this extra cost in the swap-out path. This series
>>> not only eliminates the regression but makes it faster to swap out 64K mTHP
>>> vs 4K small folios.
>>>
>>> - Memory fragmentation avoidance: If we can avoid splitting a large folio
>>> memory is less likely to become fragmented, making it easier to re-allocate
>>> a large folio in future.
>>>
>>> - Performance: Enables a separate series [4] to swap-in whole mTHPs, which
>>> means we won't lose the TLB-efficiency benefits of mTHP once the memory has
>>> been through a swap cycle.
>>>
>>> I've done what I thought was the smallest change possible, and as a result, this
>>> approach is only employed when the swap is backed by a non-rotating block device
>>> (just as PMD-sized THP is supported today). Discussion against the RFC concluded
>>> that this is sufficient.
>>>
>>>
>>> Performance Testing
>>> ===================
>>>
>>> I've run some swap performance tests on Ampere Altra VM (arm64) with 8 CPUs. The
>>> VM is set up with a 35G block ram device as the swap device and the test is run
>>> from inside a memcg limited to 40G memory. I've then run `usemem` from
>>> vm-scalability with 70 processes, each allocating and writing 1G of memory. I've
>>> repeated everything 6 times and taken the mean performance improvement relative
>>> to 4K page baseline:
>>>
>>> | alloc size | baseline | + this series |
>>> | | v6.6-rc4+anonfolio | |
>>> |:-----------|--------------------:|--------------------:|
>>> | 4K Page | 0.0% | 1.4% |
>>> | 64K THP | -14.6% | 44.2% |
>>> | 2M THP | 87.4% | 97.7% |
>>>
>>> So with this change, the 64K swap performance goes from a 15% regression to a
>>> 44% improvement. 4K and 2M swap improves slightly too.
>>
>> I don't understand why the performance of 2M THP improves. The swap
>> entry allocation becomes a little slower. Can you provide some
>> perf-profile to root cause it?
>
> I didn't post the stdev, which is quite large (~10%), so that may explain some
> of it:
>
> | kernel | mean_rel | std_rel |
> |:---------|-----------:|----------:|
> | base-4K | 0.0% | 5.5% |
> | base-64K | -14.6% | 3.8% |
> | base-2M | 87.4% | 10.6% |
> | v4-4K | 1.4% | 3.7% |
> | v4-64K | 44.2% | 11.8% |
> | v4-2M | 97.7% | 13.3% |
>
> Regardless, I'll do some perf profiling and post results shortly.

I did a lot more runs (24 for each config) and meaned them to try to remove the
noise in the measurements. It's now only showing a 4% improvement for 2M. So I
don't think the 2M improvement is real:

| kernel | mean_rel | std_rel |
|:---------|-----------:|----------:|
| base-4K | 0.0% | 3.2% |
| base-64K | -9.1% | 10.1% |
| base-2M | 88.9% | 6.8% |
| v4-4K | 0.5% | 3.1% |
| v4-64K | 44.7% | 8.3% |
| v4-2M | 93.3% | 7.8% |

Looking at the perf data, the only thing that sticks out is that a big chunk of
time is spent in during contpte_convert(), called as a result of
try_to_unmap_one(). This is present in both the before and after configs.

This is an arm64 function to "unfold" contpte mappings. Essentially, the PMD is
being split during shrink_folio_list() with TTU_SPLIT_HUGE_PMD, meaning the
THPs are PTE-mapped in contpte blocks. Then we are unmapping each pte one-by-one
which means the contpte block needs to be unfolded. I think try_to_unmap_one()
could potentially be optimized to batch unmap a contiguously mapped folio and
avoid this unfold. But that would be an independent and separate piece of work.

>
>>
>> --
>> Best Regards,
>> Huang, Ying
>>
>>> This test also acts as a good stress test for swap and, more generally mm. A
>>> couple of existing bugs were found as a result [5] [6].
>>>
>>>
>>> ---
>>> The series applies against mm-unstable (d7182786dd0a). Although I've
>>> additionally been running with a couple of extra fixes to avoid the issues at
>>> [6].
>>>
>>>
>>> Changes since v3 [3]
>>> ====================
>>>
>>> - Renamed SWAP_NEXT_NULL -> SWAP_NEXT_INVALID (per Huang, Ying)
>>> - Simplified max offset calculation (per Huang, Ying)
>>> - Reinstated struct percpu_cluster to contain per-cluster, per-order `next`
>>> offset (per Huang, Ying)
>>> - Removed swap_alloc_large() and merged its functionality into
>>> scan_swap_map_slots() (per Huang, Ying)
>>> - Avoid extra cost of folio ref and lock due to removal of CLUSTER_FLAG_HUGE
>>> by freeing swap entries in batches (see patch 2) (per DavidH)
>>> - vmscan splits folio if its partially mapped (per Barry Song, DavidH)
>>> - Avoid splitting in MADV_PAGEOUT path (per Barry Song)
>>> - Dropped "mm: swap: Simplify ssd behavior when scanner steals entry" patch
>>> since it's not actually a problem for THP as I first thought.
>>>
>>>
>>> Changes since v2 [2]
>>> ====================
>>>
>>> - Reuse scan_swap_map_try_ssd_cluster() between order-0 and order > 0
>>> allocation. This required some refactoring to make everything work nicely
>>> (new patches 2 and 3).
>>> - Fix bug where nr_swap_pages would say there are pages available but the
>>> scanner would not be able to allocate them because they were reserved for the
>>> per-cpu allocator. We now allow stealing of order-0 entries from the high
>>> order per-cpu clusters (in addition to exisiting stealing from order-0
>>> per-cpu clusters).
>>>
>>>
>>> Changes since v1 [1]
>>> ====================
>>>
>>> - patch 1:
>>> - Use cluster_set_count() instead of cluster_set_count_flag() in
>>> swap_alloc_cluster() since we no longer have any flag to set. I was unable
>>> to kill cluster_set_count_flag() as proposed against v1 as other call
>>> sites depend explicitly setting flags to 0.
>>> - patch 2:
>>> - Moved large_next[] array into percpu_cluster to make it per-cpu
>>> (recommended by Huang, Ying).
>>> - large_next[] array is dynamically allocated because PMD_ORDER is not
>>> compile-time constant for powerpc (fixes build error).
>>>
>>>
>>> [1] https://lore.kernel.org/linux-mm/20231010142111.3997780-1-ryan.roberts@arm.com/
>>> [2] https://lore.kernel.org/linux-mm/20231017161302.2518826-1-ryan.roberts@arm.com/
>>> [3] https://lore.kernel.org/linux-mm/20231025144546.577640-1-ryan.roberts@arm.com/
>>> [4] https://lore.kernel.org/linux-mm/20240304081348.197341-1-21cnbao@gmail.com/
>>> [5] https://lore.kernel.org/linux-mm/20240311084426.447164-1-ying.huang@intel.com/
>>> [6] https://lore.kernel.org/linux-mm/79dad067-1d26-4867-8eb1-941277b9a77b@arm.com/
>>>
>>> Thanks,
>>> Ryan
>>>
>>>
>>> Ryan Roberts (6):
>>> mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags
>>> mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()
>>> mm: swap: Simplify struct percpu_cluster
>>> mm: swap: Allow storage of all mTHP orders
>>> mm: vmscan: Avoid split during shrink_folio_list()
>>> mm: madvise: Avoid split during MADV_PAGEOUT and MADV_COLD
>>>
>>> include/linux/pgtable.h | 28 ++++
>>> include/linux/swap.h | 33 +++--
>>> mm/huge_memory.c | 3 -
>>> mm/internal.h | 48 +++++++
>>> mm/madvise.c | 101 ++++++++------
>>> mm/memory.c | 13 +-
>>> mm/swapfile.c | 298 ++++++++++++++++++++++------------------
>>> mm/vmscan.c | 9 +-
>>> 8 files changed, 332 insertions(+), 201 deletions(-)
>>>
>>> --
>>> 2.25.1
>