RAM 变得非常慢，写入速度低且延迟高

Question

这更像是一条长评论，而不是答案。也许它会激发其他建议。基本思想是保留和分配内存，这样 sysbench 就必须使用不同的内存区域作为其内存块缓冲区，以观察整个内存的性能是否相同。

测试系统在 4X8G DIMM 上仅有 32G 内存。

首先，只需正常运行 sysbench，但增加测试持续时间，以便在执行时获取一些信息：

doug@s19:~/c$ sysbench --test=memory --memory-block-size=4G --memory-total-size=512G --num-threads=1 --time=60 run
WARNING: the --test option is deprecated. You can pass a script name or path on the command line without any options.
sysbench 1.0.18 (using system LuaJIT 2.1.0-beta3)

Running the test with following options:
Number of threads: 1
Initializing random number generator from current time


Running memory speed test with the following options:
  block size: 4194304KiB
  total size: 524288MiB
  operation: write
  scope: global

Initializing worker threads...

Threads started!

Total operations: 128 (    3.05 per second)

524288.00 MiB transferred (12509.51 MiB/sec)


General statistics:
    total time:                          41.9101s
    total number of events:              128

Latency (ms):
         min:                                  327.22
         avg:                                  327.42
         max:                                  328.09
         95th percentile:                      325.98
         sum:                                41909.72

Threads fairness:
    events (avg/stddev):           128.0000/0.00
    execution time (avg/stddev):   41.9097/0.00

在测试执行的 41 秒内，我在另一个终端执行了以下操作：

doug@s19:~$ ps aux | grep sysbench
doug       10489 92.2 12.8 4227496 4204436 pts/1 Sl+  09:48   0:03 sysbench --test=memory --memory-block-size=4G --memory-total-size=512G --num-threads=1 --time=60 run
doug       10492  0.0  0.0   9040   732 pts/2    S+   09:48   0:00 grep --color=auto sysbench
doug@s19:~$ pmap 10489 | grep anon | grep " 4195"
00007f25cbb5a000 4195456K rw---   [ anon ]

我把整个循环重复了几次：

00007fa4bd7b4000 4195456K rw---   [ anon ]
00007f32c9878000 4195456K rw---   [ anon ]
00007f4837211000 4195456K rw---   [ anon ]

这表明我们确实不知道使用了什么内存或映射回 DIMM。无论如何，让我们继续。

编辑：通过禁用地址空间布局随机化（ASLR），可以使上述操作可重复：

sudo sysctl kernel.randomize_va_space=0

现在，保留一些内存块，以强制将 sysbench 缓冲区分配到其他地方。有人可能有更好的主意，但我写了一个程序：

doug@s19:~/c$ cat reservem.c
/*****************************************************************************
*
* reservem.c 2021.07.20 Smythies
*       allocate a chunk of memory for a while.
*       current use is to force another program to use a different area
*       of memory.
*       see also: https://askubuntu.com/questions/1352756/ram-has-become-very-slow-low-write-speed-and-high-latency
*       see slao testm.c from which this code started.
*
*****************************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(){
   char *fptr;
   long i, k;

/* Adjust as needed for use requirements */
   i = 8589934592;

   if(( fptr = (char *)malloc(i)) == NULL){
      printf("reservem: memory allocation failed, Exiting...\n");
      exit(-1);
   }
   while (( fptr == NULL) && (i > 0));
   for(k = 0; k < i; k++){   /* so that the memory really gets allocated and not just reserved */
      fptr[k] = (char) (k & 255);
   } /* endfor */
   printf("reservem: memory reserved and allocated. now sleeping...\n");
   sleep(180);  /* so other tests and observation can be done. Adjust as required. */
   free(fptr);
   printf("reservem: memory has been set free. Done and exiting...\n");
   return(0);
} /* endprogram */

编译它：

doug@s19:~/c$ cc reservem.c -o reservem

然后运行它，然后在保留和分配内存块时重复前面的步骤。得到：

doug@s19:~$ ps aux | grep sysbench
doug       11324 93.8 12.8 4227496 4204340 pts/1 Sl+  13:58   0:09 sysbench --test=memory --memory-block-size=4G --memory-total-size=512G --num-threads=1 --time=60 run
doug       11327  0.0  0.0   9040   664 pts/2    S+   13:59   0:00 grep --color=auto sysbench
doug@s19:~$ pmap 11324 | grep anon | grep " 4195"
00007fc5f2df8000 4195456K rw---   [ anon ]

对于保留的内存：

doug@s19:~$ ps aux | grep reservem
doug       11314 55.0 25.6 8391108 8389584 pts/0 S+   13:57   0:11 ./reservem
doug       11318  0.0  0.0   9040   740 pts/2    S+   13:57   0:00 grep --color=auto reservem
doug@s19:~$ pmap 11314 | grep anon | grep " 8388612K"
00007f11a6bfc000 8388612K rw---   [ anon ]

和：

524288.00 MiB transferred (12499.79 MiB/sec)

同样地，对于保留和分配2倍和3倍的8G：

doug@s19:~$ ps aux | grep reservem
doug       11335 85.0 25.6 8391108 8389704 pts/0 S    14:07   0:11 ./reservem
doug       11336 92.0 25.6 8391108 8389672 pts/0 S    14:07   0:11 ./reservem
doug       11340  0.0  0.0   9040   736 pts/2    S+   14:08   0:00 grep --color=auto reservem
doug@s19:~$ pmap 11335 | grep anon | grep " 8388612K"
00007fae2ce3f000 8388612K rw---   [ anon ]
doug@s19:~$ pmap 11336 | grep anon | grep " 8388612K"
00007f20cb627000 8388612K rw---   [ anon ]
doug@s19:~$ ps aux | grep sysbench
doug       11347 96.6 12.8 4227496 4204468 pts/1 Sl+  14:08   0:12 sysbench --test=memory --memory-block-size=4G --memory-total-size=512G --num-threads=1 --time=60 run
doug       11350  0.0  0.0   9040   740 pts/2    S+   14:08   0:00 grep --color=auto sysbench
doug@s19:~$ pmap 11347 | grep anon | grep " 4195"
00007f37dbe3c000 4195456K rw---   [ anon ]

和：

524288.00 MiB transferred (12521.74 MiB/sec)

3倍：

doug@s19:~$ ps aux | grep reservem
doug       11388  100 21.0 8391108 6889064 pts/0 R    14:12   0:09 ./reservem
doug       11389  103 19.2 8391108 6292368 pts/0 R    14:12   0:08 ./reservem
doug       11390  100 16.3 8391108 5334328 pts/0 R    14:12   0:07 ./reservem
doug       11392  0.0  0.0   9040   724 pts/2    S+   14:12   0:00 grep --color=auto reservem
doug@s19:~$ pmap 11388 | grep anon | grep " 8388612K"
00007f2b83d2d000 8388612K rw---   [ anon ]
doug@s19:~$ pmap 11389 | grep anon | grep " 8388612K"
00007f2921e0c000 8388612K rw---   [ anon ]
doug@s19:~$ pmap 11390 | grep anon | grep " 8388612K"
00007f2a23f2b000 8388612K rw---   [ anon ]
doug@s19:~$ ps aux | grep sysbench
doug       11402  107 12.8 4227496 4204420 pts/1 Sl+  14:12   0:07 sysbench --test=memory --memory-block-size=4G --memory-total-size=512G --num-threads=1 --time=60 run
doug       11405  0.0  0.0   9040   672 pts/2    S+   14:12   0:00 grep --color=auto sysbench
doug@s19:~$ pmap 11402 | grep anon | grep " 4195"
00007fe64b54b000 4195456K rw---   [ anon ]

和：

524288.00 MiB transferred (12504.34 MiB/sec)

作为参考，该系统的性能与内存块大小的关系如下：

Block-size: performance (MiB/sec):
256            2750.99
512            4937.70
  1K           8216.82
  2K          12290.93
  4K          16334.92
  8K          19498.37
 16K          21663.68
 32K          22514.94
 64K          23372.45
128K          23815.14
256K          23967.98
512K          24126.43
  1M          24226.70
  2M          24279.93
  4M          24310.19
  8M          23632.07
 16M          20622.04
 32M          16149.71
 64M          14206.06
128M          13303.15
256M          12853.12
512M          12720.34
  1G          12584.68
  2G          12538.66
  4G          12502.66
  8G          12490.63
 16G          12482.25

Answer 1