Re: [PATCH v8 00/10] Multi-size THP for anonymous memory

Ryan Roberts
2023-12-06T10:08:47Z

On 05/12/2023 14:19, Kefeng Wang wrote:
>
>
> On 2023/12/4 18:20, Ryan Roberts wrote:
>> Hi All,
>>
>> A new week, a new version, a new name... This is v8 of a series to implement
>> multi-size THP (mTHP) for anonymous memory (previously called "small-sized THP"
>> and "large anonymous folios"). Matthew objected to "small huge" so hopefully
>> this fares better.
>>
>> The objective of this is to improve performance by allocating larger chunks of
>> memory during anonymous page faults:
>>
>> 1) Since SW (the kernel) is dealing with larger chunks of memory than base
>>     pages, there are efficiency savings to be had; fewer page faults, batched PTE
>>     and RMAP manipulation, reduced lru list, etc. In short, we reduce kernel
>>     overhead. This should benefit all architectures.
>> 2) Since we are now mapping physically contiguous chunks of memory, we can take
>>     advantage of HW TLB compression techniques. A reduction in TLB pressure
>>     speeds up kernel and user space. arm64 systems have 2 mechanisms to coalesce
>>     TLB entries; "the contiguous bit" (architectural) and HPA (uarch).
>>
>> This version changes the name and tidies up some of the kernel code and test
>> code, based on feedback against v7 (see change log for details).
>>
>> By default, the existing behaviour (and performance) is maintained. The user
>> must explicitly enable multi-size THP to see the performance benefit. This is
>> done via a new sysfs interface (as recommended by David Hildenbrand - thanks to
>> David for the suggestion)! This interface is inspired by the existing
>> per-hugepage-size sysfs interface used by hugetlb, provides full backwards
>> compatibility with the existing PMD-size THP interface, and provides a base for
>> future extensibility. See [8] for detailed discussion of the interface.
>>
>> This series is based on mm-unstable (715b67adf4c8).
>>
>>
>> Prerequisites
>> =============
>>
>> Some work items identified as being prerequisites are listed on page 3 at [9].
>> The summary is:
>>
>> | item                          | status                  |
>> |:------------------------------|:------------------------|
>> | mlock                         | In mainline (v6.7)      |
>> | madvise                       | In mainline (v6.6)      |
>> | compaction                    | v1 posted [10]          |
>> | numa balancing                | Investigated: see below |
>> | user-triggered page migration | In mainline (v6.7)      |
>> | khugepaged collapse           | In mainline (NOP)       |
>>
>> On NUMA balancing, which currently ignores any PTE-mapped THPs it encounters,
>> John Hubbard has investigated this and concluded that it is A) not clear at the
>> moment what a better policy might be for PTE-mapped THP and B) questions whether
>> this should really be considered a prerequisite given no regression is caused
>> for the default "multi-size THP disabled" case, and there is no correctness
>> issue when it is enabled - its just a potential for non-optimal performance.
>>
>> If there are no disagreements about removing numa balancing from the list (none
>> were raised when I first posted this comment against v7), then that just leaves
>> compaction which is in review on list at the moment.
>>
>> I really would like to get this series (and its remaining comapction
>> prerequisite) in for v6.8. I accept that it may be a bit optimistic at this
>> point, but lets see where we get to with review?
>>
>>
>> Testing
>> =======
>>
>> The series includes patches for mm selftests to enlighten the cow and khugepaged
>> tests to explicitly test with multi-size THP, in the same way that PMD-sized
>> THP is tested. The new tests all pass, and no regressions are observed in the mm
>> selftest suite. I've also run my usual kernel compilation and java script
>> benchmarks without any issues.
>>
>> Refer to my performance numbers posted with v6 [6]. (These are for multi-size
>> THP only - they do not include the arm64 contpte follow-on series).
>>
>> John Hubbard at Nvidia has indicated dramatic 10x performance improvements for
>> some workloads at [11]. (Observed using v6 of this series as well as the arm64
>> contpte series).
>>
>> Kefeng Wang at Huawei has also indicated he sees improvements at [12] although
>> there are some latency regressions also.
>
> Hi Ryan,
>
> Here is some test results based on v6.7-rc1 +
> [PATCH v7 00/10] Small-sized THP for anonymous memory +
> [PATCH v2 00/14] Transparent Contiguous PTEs for User Mappings
>
> case1: basepage 64K
> case2: basepage 4K + thp=64k + PAGE_ALLOC_COSTLY_ORDER = 3
> case3: basepage 4K + thp=64k + PAGE_ALLOC_COSTLY_ORDER = 4

Thanks for sharing these results. With the exception of a few outliers, It looks
like the ~rough conclusion is that bandwidth improves, but not as much as 64K
base pages, and latency regresses, but also not as much as 64K base pages?

I expect that over time, as we add more optimizations, we will get bandwidth
closer to 64K base pages; one crucial one is getting executable file-backed
memory into contpte mappings, for example.

It's probably not time to switch PAGE_ALLOC_COSTLY_ORDER quite yet; but
something to keep an eye on and consider down the road?

Thanks,
Ryan

>
> The results is compared with basepage 4K on Kunpeng920.
>
> Note,
> - The test based on ext4 filesystem and THP=2M is disabled.
> - The results were not analyzed, it is for reference only,
>   as some values of test items are not consistent.
>
> 1) Unixbench 1core
> Index_Values_1core                       case1       case2    case3
> Dhrystone_2_using_register_variables     0.28%      0.39%     0.17%
> Double-Precision_Whetstone              -0.01%      0.00%     0.00%
> Execl_Throughput                        *21.13%*    2.16%     3.01%
> File_Copy_1024_bufsize_2000_maxblocks   -0.51%     *8.33%*   *8.76%*
> File_Copy_256_bufsize_500_maxblocks      0.78%     *11.89%*  *10.85%*
> File_Copy_4096_bufsize_8000_maxblocks    7.42%      7.27%    *10.66%*
> Pipe_Throughput                         -0.24%     *6.82%*   *5.08%*
> Pipe-based_Context_Switching             1.38%     *13.49%*  *9.91%*
> Process_Creation                        *32.46%*    4.30%    *8.54%*
> Shell_Scripts_(1_concurrent)            *31.67%*    1.92%     2.60%
> Shell_Scripts_(8_concurrent)            *40.59%*    1.30%    *5.29%*
> System_Call_Overhead                     3.92%     *8.13%     2.96%
>
> System_Benchmarks_Index_Score           10.66%      5.39%     5.58%
>
> For 1core,
> - case1 wins on Execl_Throughput/Process_Creation/Shell_Scripts
>   a lot, and score higher 10.66% vs basepage 4K.
> - case2/3 wins on File_Copy/Pipe and score higher 5%+ than basepage 4K,
>   also case3 looks better on Shell_Scripts_(8_concurrent) than case2.
>
> 2) Unixbench 128core
> Index_Values_128core                    case1     case2     case3
> Dhrystone_2_using_register_variables    2.07%    -0.03%    -0.11%
> Double-Precision_Whetstone             -0.03%     0.00%    0.00%
> Execl_Throughput                       *39.28%*  -4.23%    1.93%
> File_Copy_1024_bufsize_2000_maxblocks   5.46%     1.30%    4.20%
> File_Copy_256_bufsize_500_maxblocks    -8.89%    *6.56%   *5.02%*
> File_Copy_4096_bufsize_8000_maxblocks   3.43%   *-5.46%*   0.56%
> Pipe_Throughput                         3.80%    *7.69%   *7.80%*
> Pipe-based_Context_Switching           *7.62%*    0.95%    4.69%
> Process_Creation                       *28.11%*  -2.79%    2.40%
> Shell_Scripts_(1_concurrent)           *39.68%*   1.86%   *5.30%*
> Shell_Scripts_(8_concurrent)           *41.35%*   2.49%   *7.16%*
> System_Call_Overhead                   -1.55%    -0.04%   *8.23%*
>
> System_Benchmarks_Index_Score          12.08%     0.63%    3.88%
>
> For 128core,
> - case1 wins on Execl_Throughput/Process_Creation/Shell_Scripts
>   a lot, also good at Pipe-based_Context_Switching, and score higher
>   12.08% vs basepage 4K.
> - case2/case3 wins on File_Copy_256/Pipe_Throughput, but case2 is
>   not better than basepage 4K, case3 wins 3.88%.
>
> 3) Lmbench Processor_processes
> Processor_Processes    case1      case2      case3
> null_call              1.76%      0.40%     0.65%
> null_io               -0.76%     -0.38%    -0.23%
> stat                 *-16.09%*  *-12.49%*   4.22%
> open_close            -2.69%      4.51%     3.21%
> slct_TCP              -0.56%      0.00%    -0.44%
> sig_inst              -1.54%      0.73%     0.70%
> sig_hndl              -2.85%      0.01%     1.85%
> fork_proc            *23.31%*     8.77%    -5.42%
> exec_proc            *13.22%*    -0.30%     1.09%
> sh_proc              *14.04%*    -0.10%     1.09%
>
> - case1 is much better than basepage 4K, same as Unixbench test,
>   case2 is better on fork_proc, but case3 is worse
> - note: the variance of fork/exec/sh is bigger than others
>
> 4) Lmbench Context_switching_ctxsw
> Context_switching_ctxsw  case1     case2         case3
> 2p/0K                   -12.16%    -5.29%       -1.86%
> 2p/16K                  -11.26%    -3.71%       -4.53%
> 2p/64K                  -2.60%      3.84%       -1.98%
> 8p/16K                  -7.56%     -1.21%       -0.88%
> 8p/64K                   5.10%      4.88%        1.19%
> 16p/16K                 -5.81%     -2.44%       -3.84%
> 16p/64K                  4.29%     -1.94%       -2.50%
> - case1/2/3 worse than basepage 4K and case1 is the worst.
>
> 4) Lmbench Local_latencies
> Local_latencies      case1      case2     case3
> Pipe                -9.23%      0.58%    -4.34%
> AF_UNIX             -5.34%     -1.76%     3.03%
> UDP                 -6.70%     -5.96%    -9.81%
> TCP                 -7.95%     -7.58%    -5.63%
> TCP_conn            -213.99%   -227.78%  -659.67%
> - TCP_conn is very unreliable, ignore it
> - case1/2/3 slower than basepage 4K
>
> 5) Lmbench File_&_VM_latencies
> File_&_VM_latencies    case1     case2        case3
> 10K_File_Create        2.60%    -0.52%         2.66%
> 10K_File_Delete       -2.91%    -5.20%        -2.11%
> 10K_File_Create       10.23%     1.18%         0.12%
> 10K_File_Delete      -17.76%    -2.97%        -1.49%
> Mmap_Latency         *63.05%*    2.57%        -0.96%
> Prot_Fault            10.41%    -3.21%       *-19.11%*
> Page_Fault          *-132.01%*   2.35%        -0.79%
> 100fd_selct          -1.20%      0.10%         0.31%
> - case1 is very good at Mmap_Latency and not good at Page_fault
> - case2/3 slower on Prot_Faul/10K_FILE_Delete vs basepage 4k,
>   the rest doesn't look much different.
>
> 6) Lmbench Local_bandwidths
> Local_bandwidths    case1   case2       case3
> Pipe               265.22%   15.44%     11.33%
> AF_UNIX            13.41%   -2.66%      2.63%
> TCP               -1.30%     25.90%     2.48%
> File_reread        14.79%    31.52%    -14.16%
> Mmap_reread        27.47%    49.00%    -0.11%
> Bcopy(libc)        2.58%     2.45%      2.46%
> Bcopy(hand)        25.78%    22.56%     22.68%
> Mem_read           38.26%    36.80%     36.49%
> Mem_write          10.93%    3.44%      3.12%
>
> - case1 is very good at bandwidth, case2 is better than basepage 4k
>   but lower than case1, case3 is bad at File_reread
>
> 7)Lmbench Memory_latencies
> Memory_latencies    case1     case2     case3
> L1_$                0.02%     0.00%    -0.03%
> L2_$               -1.56%    -2.65%    -1.25%
> Main_mem           50.82%     32.51%    33.47%
> Rand_mem           15.29%    -8.79%    -8.80%
>
> - case1 also good at Main/Rand mem access latencies,
> - case2/case3 is better at Main_mem, but worse at Rand_mem.
>
> Tested-by: Kefeng Wang <wangkefeng.wang@huawei.com>