DAX文件系统大页映射性能测试方法

DAX文件系统+mmap()文件接口,可以将持久型内存(persistent memory)直接映射到用户态,我预计这在未来将成为一种在系统编程时操作PM的标准套路。本文将通过实验验证DAX文件系统2M大页映射,并指出测试性能的方法。

1. 如何配置

[1]给出了比较详细的配置说明,这里避免过多重复,简单总结一下。

根据[1]所述,内核支持DAX的文件系统ext4和XFS现在支持2MB大小的hugepage了,但要使用这个特性,需要满足3个条件:

  1. mmap()调用必须最少映射 2 MiB;
  2. 文件系统块的最少以2 MiB被分配;
  3. 文件系统块必须和mmap()调用有相同的对齐量。

其中,第1点(在用户态的调用)挺容易达到的,第2、3点得益于当年为了支持RAID所提出的特性,ext4和XFS也支持从底层分配一定大小和一定对齐量的块。关于怎么快速验证2MB大页映射,[1]中也以fallocate一个1GB大小的文件进行了说明。本文更进一步,验证对于一个文件,怎样创建它、怎样映射它才能利用到2MB大页映射。

2. 监测2M大页page fault的结果

我们要验证大页映射是否成功,可以对大页fault成功与否进行监测。[1]中利用了内核函数tracepoint机制,对构建pmd页表项的dax_pmd_fault_done这个tracepoint进行监测。这里我们用的是bpftrace这个工具,它可以更方便地监测tracepoint。步骤如下:

**Step 1.**根据文档[1]和bpftrace文档[2],我们可以首先查看dax_pmd_fault_done这个tracepoint的相关信息:

$ sudo cat /sys/kernel/debug/tracing/events/fs_dax/dax_pmd_fault_done/format
name: dax_pmd_fault_done
ID: 888
format:
        field:unsigned short common_type;       offset:0;       size:2; signed:0;
        field:unsigned char common_flags;       offset:2;       size:1; signed:0;
        field:unsigned char common_preempt_count;       offset:3;       size:1; signed:0;
        field:int common_pid;   offset:4;       size:4; signed:1;

        field:unsigned long ino;        offset:8;       size:8; signed:0;
        field:unsigned long vm_start;   offset:16;      size:8; signed:0;
        field:unsigned long vm_end;     offset:24;      size:8; signed:0;
        field:unsigned long vm_flags;   offset:32;      size:8; signed:0;
        field:unsigned long address;    offset:40;      size:8; signed:0;
        field:unsigned long pgoff;      offset:48;      size:8; signed:0;
        field:unsigned long max_pgoff;  offset:56;      size:8; signed:0;
        field:dev_t dev;        offset:64;      size:4; signed:0;
        field:unsigned int flags;       offset:68;      size:4; signed:0;
        field:int result;       offset:72;      size:4; signed:1;

print fmt: "dev %d:%d ino %#lx %s %s address %#lx vm_start %#lx vm_end %#lx pgoff %#lx max_pgoff %#lx %s", ((unsigned int) ((REC->dev) >> 20)), ((unsigned int) ((REC->dev) & ((1U << 20) - 1))), REC->ino, REC->vm_flags & 0x00000008 ? "shared" : "private", __print_flags(REC->flags, "|", { 0x01, "WRITE" }, { 0x02, "MKWRITE" }, { 0x04, "ALLOW_RETRY" }, { 0x08, "RETRY_NOWAIT" }, { 0x10, "KILLABLE" }, { 0x20, "TRIED" }, { 0x40, "USER" }, { 0x80, "REMOTE" }, { 0x100, "INSTRUCTION" }), REC->address, REC->vm_start, REC->vm_end, REC->pgoff, REC->max_pgoff, __print_flags(REC->result, "|", { 0x0001, "OOM" }, { 0x0002, "SIGBUS" }, { 0x0004, "MAJOR" }, { 0x0008, "WRITE" }, { 0x0010, "HWPOISON" }, { 0x0020, "HWPOISON_LARGE" }, { 0x0040, "SIGSEGV" }, { 0x0100, "NOPAGE" }, { 0x0200, "LOCKED" }, { 0x0400, "RETRY" }, { 0x0800, "FALLBACK" }, { 0x1000, "DONE_COW" }, { 0x2000, "NEEDDSYNC" })

Step 2. 然后用一行bpftrace命令判断是否使用大页成功:

sudo bpftrace -e ' tracepoint:fs_dax:dax_pmd_fault_done {printf("dax_pmd_fault_done result: 0x%x\n", args->result);@[args->result]=count()}'

注意这行命令中,我们打印的是result这个信息,它一般会在2MB对齐页发生page fault时打印出0x1000x800两个结构。这两个结构分别对应VM_FAULT_NOPAGEVM_FAULT_FALLBACK,表示缺页时2M页表项创建成功或失败。

3. 修改fio mmap引擎

fio的mmap引擎中,原本的实现是映射测试文件的全部范围:

static int fio_mmap_file(struct thread_data *td, struct fio_file *f,
             size_t length, off_t off)
{
    ...
    fmd->mmap_ptr = mmap(NULL, length, flags, shared, f->fd, off);
    if (fmd->mmap_ptr == MAP_FAILED) {
        fmd->mmap_ptr = NULL;
        td_verror(td, errno, "mmap");
        goto err;
    }
    ...
}

这里我们为了验证mmap以不同粒度映射这个文件,更改一种映射形式。我们先映射一个文件大小的匿名(用MAP_ANONYMOUS实现)区域,然后再以我们需要的映射粒度,逐一往这个区域"覆盖(用MAP_FIXED实现)"文件映射:

static int fio_mmap_file(struct thread_data *td, struct fio_file *f,
             size_t length, off_t off)
{
    ...
	  unsigned long long  align_2mb = 1024 * 1024 * 2;
    // 为了满足条件1,这里多映射2MB,然后调整fmd->mmap_ptr到一个2MB对齐的且length长的映射区
    fmd->mmap_ptr = mmap(NULL, length + align_2mb, flags, shared|MAP_ANONYMOUS , -1, 0);
    if (fmd->mmap_ptr == MAP_FAILED) {
        fmd->mmap_ptr = NULL;
        td_verror(td, errno, "mmap");
        goto err;
    }
    fmd->mmap_ptr = (void *)((unsigned long long)fmd->mmap_ptr / align_2mb * align_2mb + align_2mb);

    // 这里可以调整我们每次映射的区域大小,比如这里是每次2MB映射这个文件,我们还会调整到
    // 每次1MB映射来验证1MB大小的映射是否可以利用2MB大页表项
    size_t map_size = 1024 * 1024 * 2;
    for (off_t i = 0; i < length - map_size; i += map_size) {
        void *new_addr = fmd->mmap_ptr + i;
        void *res = mmap(new_addr, map_size, flags, shared|MAP_FIXED, f->fd, off + i);
        if (res == MAP_FAILED) {
            printf("error: %s\n", strerror(errno));
        }
    }
    ...
}

在上述代码map_size分别设置成1024 * 1024 * 21024 * 1024 * 1 (即2MB或1MB映射文件),分别重新编译fio。用如下fio脚本测试:

# test.fio
[global]
thread
group_reporting
overwrite=1
thinktime=0
sync=0
direct=0
ioengine=mmap
filename=/pmem6/testdata
size=100GB
time_based
runtime=10
[Random_Read]
bs=4k
numjobs=1
iodepth=1
rw=randread

4. 测试及结果分析

4.1. 怎么运行测试呢:

首先,在另一个shell,用bpftrace 追踪dax_pmd_fault_done这个tracepoint的结果:

$ sudo bpftrace -e ' tracepoint:fs_dax:dax_pmd_fault_done {printf("dax_pmd_fault_done result: 0x%x\n", args->result);@[args->result]=count()}'

然后,在另一个shell,运行测试test.fio并用perf stat统计各种性能参数,之后会同时得到fio和perf的测试报告,同时bpftrace那个shell也监测到了内核tracepoint的相关情况。

$ perf stat -ddd /MY_FIO_BIN_DIR/fio test.fio

4.2. 测试结果1: 以2 MB为粒度映射文件

fio 及 perf stat 的report如下:

zjc@test_dax_hugepage $ perf stat -ddd /home/zjc/bin/fio-mod/bin/fio mmap_lat.fio
SeqR: (g=0): rw=randread, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=mmap, iodepth=1
fio-3.16
Starting 1 thread
Jobs: 1 (f=1): [r(1)][100.0%][r=1723MiB/s][r=441k IOPS][eta 00m:00s]
SeqR: (groupid=0, jobs=1): err= 0: pid=137311: Sat Nov 23 18:47:58 2019
  read: IOPS=501k, BW=1957MiB/s (2052MB/s)(19.1GiB/10001msec)
    clat (nsec): min=1116, max=1189.9k, avg=1639.48, stdev=1276.94
     lat (nsec): min=1145, max=1189.9k, avg=1666.78, stdev=1277.78
    clat percentiles (nsec):
     |  1.00th=[ 1368],  5.00th=[ 1400], 10.00th=[ 1416], 20.00th=[ 1448],
     | 30.00th=[ 1464], 40.00th=[ 1480], 50.00th=[ 1496], 60.00th=[ 1528],
     | 70.00th=[ 1560], 80.00th=[ 1624], 90.00th=[ 1976], 95.00th=[ 2352],
     | 99.00th=[ 3632], 99.50th=[ 3888], 99.90th=[11712], 99.95th=[13248],
     | 99.99th=[14400]
   bw (  MiB/s): min= 1292, max= 2088, per=100.00%, avg=1969.49, stdev=216.98, samples=19
   iops        : min=330974, max=534696, avg=504188.63, stdev=55547.02, samples=19
  lat (usec)   : 2=90.35%, 4=9.28%, 10=0.20%, 20=0.17%, 50=0.01%
  lat (msec)   : 2=0.01%
  cpu          : usr=98.13%, sys=1.86%, ctx=19, majf=0, minf=52873
  IO depths    : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
     submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     issued rwts: total=5010392,0,0,0 short=0,0,0,0 dropped=0,0,0,0
     latency   : target=0, window=0, percentile=100.00%, depth=1

Run status group 0 (all jobs):
   READ: bw=1957MiB/s (2052MB/s), 1957MiB/s-1957MiB/s (2052MB/s-2052MB/s), io=19.1GiB (20.5GB), run=10001-10001msec

Disk stats (read/write):
  pmem6: ios=0/0, merge=0/0, ticks=0/0, in_queue=0, util=0.00%

 Performance counter stats for '/home/zjc/bin/fio-mod/bin/fio mmap_lat.fio':

      13209.252198      task-clock:u (msec)       #    1.248 CPUs utilized
                 0      context-switches:u        #    0.000 K/sec
                 0      cpu-migrations:u          #    0.000 K/sec
           157,520      page-faults:u             #    0.012 M/sec
    32,808,107,404      cycles:u                  #    2.484 GHz                      (37.84%)
    13,767,631,499      instructions:u            #    0.42  insn per cycle           (45.78%)
     2,546,685,790      branches:u                #  192.796 M/sec                    (46.08%)
         3,557,939      branch-misses:u           #    0.14% of all branches          (46.44%)
     3,983,151,533      L1-dcache-loads:u         #  301.543 M/sec                    (46.74%)
       350,423,104      L1-dcache-load-misses:u   #    8.80% of all L1-dcache hits    (46.89%)
       147,288,693      LLC-loads:u               #   11.150 M/sec                    (30.98%)
         4,700,903      LLC-load-misses:u         #    6.38% of all LL-cache hits     (30.77%)
   <not supported>      L1-icache-loads:u
         5,876,206      L1-icache-load-misses:u                                       (30.69%)
     4,043,536,403      dTLB-loads:u              #  306.114 M/sec                    (30.63%)
         5,516,303      dTLB-load-misses:u        #    0.14% of all dTLB cache hits   (30.45%)
            19,166      iTLB-loads:u              #    0.001 M/sec                    (30.22%)
           120,540      iTLB-load-misses:u        #  628.93% of all iTLB cache hits   (30.10%)
   <not supported>      L1-dcache-prefetches:u
   <not supported>      L1-dcache-prefetch-misses:u

      10.580851824 seconds time elapsed

bpftrace的结果:

zjc@test_dax_hugepage $ sudo bpftrace -e ' tracepoint:fs_dax:dax_pmd_fault_done {printf("dax_pmd_fault_done result: 0x%x\n", args->result);@[args->result]=count()}'
dax_pmd_fault_done result: 0x100
dax_pmd_fault_done result: 0x100
dax_pmd_fault_done result: 0x100
...
dax_pmd_fault_done result: 0x100
dax_pmd_fault_done result: 0x100
^C

@[256]: 51199

4.3. 测试结果2: 以1MB为粒度映射文件

fio 及 perf stat 的report如下:

zjc@test_dax_hugepage $ perf stat -ddd /home/zjc/bin/fio-mod/bin/fio mmap_lat.fio
SeqR: (g=0): rw=randread, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=mmap, iodepth=1
fio-3.16
Starting 1 thread
Jobs: 1 (f=1): [r(1)][100.0%][r=885MiB/s][r=227k IOPS][eta 00m:00s]
SeqR: (groupid=0, jobs=1): err= 0: pid=136706: Sat Nov 23 18:39:56 2019
  read: IOPS=217k, BW=849MiB/s (890MB/s)(8488MiB/10001msec)
    clat (usec): min=2, max=1967, avg= 4.15, stdev= 2.82
     lat (usec): min=2, max=1967, avg= 4.18, stdev= 2.82
    clat percentiles (nsec):
     |  1.00th=[ 3440],  5.00th=[ 3600], 10.00th=[ 3696], 20.00th=[ 3792],
     | 30.00th=[ 3856], 40.00th=[ 3920], 50.00th=[ 3984], 60.00th=[ 4048],
     | 70.00th=[ 4128], 80.00th=[ 4192], 90.00th=[ 4384], 95.00th=[ 4640],
     | 99.00th=[ 8896], 99.50th=[ 9792], 99.90th=[16512], 99.95th=[17024],
     | 99.99th=[20096]
   bw (  KiB/s): min=373528, max=908128, per=99.78%, avg=867118.63, stdev=125170.53, samples=19
   iops        : min=93382, max=227032, avg=216779.63, stdev=31292.62, samples=19
  lat (usec)   : 4=54.10%, 10=45.42%, 20=0.47%, 50=0.01%, 100=0.01%
  lat (msec)   : 2=0.01%
  cpu          : usr=46.18%, sys=53.79%, ctx=89, majf=0, minf=2176958
  IO depths    : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
     submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     issued rwts: total=2172849,0,0,0 short=0,0,0,0 dropped=0,0,0,0
     latency   : target=0, window=0, percentile=100.00%, depth=1

Run status group 0 (all jobs):
   READ: bw=849MiB/s (890MB/s), 849MiB/s-849MiB/s (890MB/s-890MB/s), io=8488MiB (8900MB), run=10001-10001msec

Disk stats (read/write):
  pmem6: ios=0/0, merge=0/0, ticks=0/0, in_queue=0, util=0.00%

 Performance counter stats for '/home/zjc/bin/fio-mod/bin/fio mmap_lat.fio':

      13256.946076      task-clock:u (msec)       #    1.243 CPUs utilized
                 0      context-switches:u        #    0.000 K/sec
                 0      cpu-migrations:u          #    0.000 K/sec
         2,282,121      page-faults:u             #    0.172 M/sec
    20,141,999,012      cycles:u                  #    1.519 GHz                      (37.85%)
     6,079,376,470      instructions:u            #    0.30  insn per cycle           (45.56%)
     1,126,849,210      branches:u                #   85.001 M/sec                    (45.83%)
         6,968,748      branch-misses:u           #    0.62% of all branches          (46.12%)
     1,754,527,971      L1-dcache-loads:u         #  132.348 M/sec                    (46.36%)
       248,313,128      L1-dcache-load-misses:u   #   14.15% of all L1-dcache hits    (46.60%)
        71,871,943      LLC-loads:u               #    5.421 M/sec                    (31.13%)
         5,340,391      LLC-load-misses:u         #   14.86% of all LL-cache hits     (30.90%)
   <not supported>      L1-icache-loads:u
       184,362,644      L1-icache-load-misses:u                                       (30.77%)
     1,764,918,323      dTLB-loads:u              #  133.132 M/sec                    (30.73%)
         8,388,448      dTLB-load-misses:u        #    0.48% of all dTLB cache hits   (30.67%)
           114,007      iTLB-loads:u              #    0.009 M/sec                    (30.55%)
         3,475,836      iTLB-load-misses:u        # 3048.79% of all iTLB cache hits   (30.34%)
   <not supported>      L1-dcache-prefetches:u
   <not supported>      L1-dcache-prefetch-misses:u

      10.669283491 seconds time elapsed

bpftrace的结果:

zjc@test_dax_hugepage $ sudo bpftrace -e ' tracepoint:fs_dax:dax_pmd_fault_done {printf("dax_pmd_fault_done result: 0x%x\n", args->result);@[args->result]=count()}'
dax_pmd_fault_done result: 0x800
dax_pmd_fault_done result: 0x800
dax_pmd_fault_done result: 0x800
...
dax_pmd_fault_done result: 0x800
dax_pmd_fault_done result: 0x800
^C

@[2048]: 51199

4.4. 测试结果分析

实验环境:

项目 参数
kernel版本 4.16.0
fio版本 3.16
文件系统 ext4-dax
硬件设备 AEP (interleaved)

结果对比:

2MB mmap 1MB mmap
dax_pmd_fault_done result VM_FAULT_NOPAGE VM_FAULT_FALLBACK
随机读延迟 (us) 1.67 4.18
page-faults:u 157,520 2,282,121
dTLB-load-misses:u 5,516,303 8,388,448
iTLB-load-misses:u 120,540 3,475,836

可以发现,2MB大小的mmap调用才可以利用DAX文件系统的hugepage映射特性,在AEP上,用fio mmap引擎测试的随机4K读延迟性能提升3倍,page fault、TLB miss都显著减少了。


[1] https://nvdimm.wiki.kernel.org/2mib_fs_dax

[2] https://github.com/iovisor/bpftrace/blob/master/docs/reference_guide.md#6-tracepoint-static-tracing-kernel-level-arguments

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