cddl/contrib/dtracetoolkit/Examples/diskhits_example.txt

   1 The following is a demonstration of the diskhits command.
   2
   3
   4 Here we run diskhits on a large file, /extra1/contents with is 46 Mb, and
   5 currently hasn't been accessed (so isn't in any cache).
   6
   7 While diskhits is running, the file is grep'd in another window. This causes
   8 the entire file to be read,
   9
  10    # ./diskhits /extra1/contents
  11    Tracing... Hit Ctrl-C to end.
  12    ^C
  13    Location (KB),
  14
  15               value  ------------- Distribution ------------- count
  16                 < 0 |                                         0
  17                   0 |@@                                       47
  18                2303 |@@                                       41
  19                4606 |@@                                       41
  20                6909 |@@                                       42
  21                9212 |@@                                       41
  22               11515 |@@                                       41
  23               13818 |@@                                       42
  24               16121 |@@                                       43
  25               18424 |@@                                       42
  26               20727 |@@                                       41
  27               23030 |@@                                       41
  28               25333 |@@                                       41
  29               27636 |@@                                       41
  30               29939 |@@                                       42
  31               32242 |@@                                       44
  32               34545 |@@                                       41
  33               36848 |@@                                       41
  34               39151 |@@                                       41
  35               41454 |@@                                       41
  36               43757 |@@                                       40
  37            >= 46060 |                                         0
  38
  39    Size (KB),
  40
  41               value  ------------- Distribution ------------- count
  42                4 |                                         0
  43                   8 |                                         6
  44                  16 |                                         10
  45                  32 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@  818
  46                  64 |                                         0
  47
  48    Total RW: 46064 KB
  49
  50 Ok, so the file was read evently with each access around 32 to 63 Kb in size,
  51 and a total of 46 Mb read. This all makes sense, as it is reading the file
  52 for the first time.
  53
  54
  55
  56 Now the same file is grep'd with diskhits running, this time we can see what
  57 effect caching the file has made,
  58
  59    # ./diskhits /extra1/contents
  60    Tracing... Hit Ctrl-C to end.
  61    ^C
  62    Location (KB),
  63
  64               value  ------------- Distribution ------------- count
  65                2303 |                                         0
  66                4606 |                                         5
  67                6909 |@                                        67
  68                9212 |@@@@                                     170
  69               11515 |@@@@@                                    216
  70               13818 |@@@@@                                    224
  71               16121 |@@@@@@                                   287
  72               18424 |@@@@@                                    227
  73               20727 |@@@                                      144
  74               23030 |@@                                       75
  75               25333 |@                                        59
  76               27636 |@                                        42
  77               29939 |@                                        41
  78               32242 |@                                        44
  79               34545 |@                                        41
  80               36848 |@                                        41
  81               39151 |@                                        41
  82               41454 |@                                        41
  83               43757 |@                                        39
  84            >= 46060 |                                         0
  85
  86    Size (KB),
  87
  88               value  ------------- Distribution ------------- count
  89                   2 |                                         0
  90                   4 |@@@@@@@@@@@@@@@@@@@@@@@@@                1137
  91                   8 |@@@@@                                    211
  92                  16 |@@                                       111
  93                  32 |@@@@@@@@                                 345
  94                  64 |                                         0
  95
  96    Total RW: 29392 KB
  97
  98 The difference is dramatic. This time only 29 Mb is read, leaving around
  99 17 Mb that was read from the cache. The way the file is read differs -
 100 in the later half of the file it looks the same, but in the first half there
 101 are many more events; oddly enough, this is because the early part of the
 102 file is cached more, the extra events are likely to be much smaller in size -
 103 as indicated in the difference in the size distribution.
 104
 105 It appears that everything less that 4606 Kb has remained in the cache, with
 106 zero hits for that range.
 107