On 13/03/2024 01:15, Huang, Ying wrote: > Ryan Roberts <ryan.roberts@xxxxxxx> writes: > >> On 12/03/2024 08:49, Ryan Roberts wrote: >>> On 12/03/2024 08:01, Huang, Ying wrote: >>>> Ryan Roberts <ryan.roberts@xxxxxxx> 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. > > Thanks for more data and detailed explanation. And thanks for your review! I'll address all your comments (and any others that I get in the meantime) and repost after the merge window. It would be great if we can get this in for v6.10. > > -- > Best Regards, > Huang, Ying
