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