1Demonstrations of dcstat, the Linux eBPF/bcc version.
2
3
4dcstat shows directory entry cache (dcache) statistics. For example:
5
6# ./dcstat
7TIME         REFS/s   SLOW/s   MISS/s     HIT%
808:11:47:      2059      141       97    95.29
908:11:48:     79974      151      106    99.87
1008:11:49:    192874      146      102    99.95
1108:11:50:      2051      144      100    95.12
1208:11:51:     73373    17239    17194    76.57
1308:11:52:     54685    25431    25387    53.58
1408:11:53:     18127     8182     8137    55.12
1508:11:54:     22517    10345    10301    54.25
1608:11:55:      7524     2881     2836    62.31
1708:11:56:      2067      141       97    95.31
1808:11:57:      2115      145      101    95.22
19
20The output shows the total references per second ("REFS/s"), the number that
21took a slower code path to be processed ("SLOW/s"), the number of dcache misses
22("MISS/s"), and the hit ratio as a percentage. By default, an interval of 1
23second is used.
24
25At 08:11:49, there were 192 thousand references, which almost entirely hit
26from the dcache, with a hit ration of 99.95%. A little later, starting at
2708:11:51, a workload began that walked many uncached files, reducing the hit
28ratio to 53%, and more importantly, a miss rate of over 10 thousand per second.
29
30
31Here's an interesting workload:
32
33# ./dcstat
34TIME         REFS/s   SLOW/s   MISS/s     HIT%
3508:15:53:    250683      141       97    99.96
3608:15:54:    266115      145      101    99.96
3708:15:55:    268428      141       97    99.96
3808:15:56:    260389      143       99    99.96
39
40It's a 99.96% hit ratio, and these are all negative hits: accessing a file that
41does not exist. Here's the C program that generated the workload:
42
43# cat -n badopen.c
44     1	#include <sys/types.h>
45     2	#include <sys/stat.h>
46     3	#include <fcntl.h>
47     4
48     5	int
49     6	main(int argc, char *argv[])
50     7	{
51     8	    int fd;
52     9	    while (1) {
53    10	        fd = open("bad", O_RDONLY);
54    11	    }
55    12	    return 0;
56    13	}
57
58This is a simple workload generator than tries to open a missing file ("bad")
59as quickly as possible.
60
61
62Lets see what happens if the workload attempts to open a different filename
63each time (which is also a missing file), using the following C code:
64
65# cat -n badopen2.c
66     1	#include <sys/types.h>
67     2	#include <sys/stat.h>
68     3	#include <fcntl.h>
69     4	#include <stdio.h>
70     5
71     6	int
72     7	main(int argc, char *argv[])
73     8	{
74     9	    int fd, i = 0;
75    10	    char buf[128] = {};
76    11
77    12	    while (1) {
78    13	        sprintf(buf, "bad%d", i++);
79    14	        fd = open(buf, O_RDONLY);
80    15	    }
81    16	    return 0;
82    17	}
83
84Here's dcstat:
85
86# ./dcstat
87TIME         REFS/s   SLOW/s   MISS/s     HIT%
8808:18:52:    241131   237544   237505     1.51
8908:18:53:    238210   236323   236278     0.82
9008:18:54:    235259   233307   233261     0.85
9108:18:55:    233144   231256   231214     0.83
9208:18:56:    231981   230097   230053     0.83
93
94
95dcstat also supports an optional interval and optional count. For example,
96printing 5 second summaries 3 times:
97
98# ./dcstat 5 3
99TIME         REFS/s   SLOW/s   MISS/s     HIT%
10008:20:03:      2085      143       99    95.23
10108:20:08:      2077      143       98    95.24
10208:20:14:      2071      144      100    95.15
103
104
105USAGE message:
106
107# ./dcstat -h
108USAGE: ./dcstat [interval [count]]
109