| Brendan Gregg | 9fa1562 | 2015-09-21 15:51:11 -0700 | [diff] [blame] | 1 | Demonstrations of biolatency, the Linux eBPF/bcc version. |
| 2 | |
| 3 | |
| 4 | biolatency traces block device I/O (disk I/O), and records the distribution |
| 5 | of I/O latency (time), printing this as a histogram when Ctrl-C is hit. |
| 6 | For example: |
| 7 | |
| Alexei Starovoitov | bdf0773 | 2016-01-14 10:09:20 -0800 | [diff] [blame] | 8 | # ./biolatency |
| Brendan Gregg | 9fa1562 | 2015-09-21 15:51:11 -0700 | [diff] [blame] | 9 | Tracing block device I/O... Hit Ctrl-C to end. |
| 10 | ^C |
| 11 | usecs : count distribution |
| 12 | 0 -> 1 : 0 | | |
| 13 | 2 -> 3 : 0 | | |
| 14 | 4 -> 7 : 0 | | |
| 15 | 8 -> 15 : 0 | | |
| 16 | 16 -> 31 : 0 | | |
| 17 | 32 -> 63 : 0 | | |
| 18 | 64 -> 127 : 1 | | |
| 19 | 128 -> 255 : 12 |******** | |
| 20 | 256 -> 511 : 15 |********** | |
| 21 | 512 -> 1023 : 43 |******************************* | |
| 22 | 1024 -> 2047 : 52 |**************************************| |
| 23 | 2048 -> 4095 : 47 |********************************** | |
| 24 | 4096 -> 8191 : 52 |**************************************| |
| 25 | 8192 -> 16383 : 36 |************************** | |
| 26 | 16384 -> 32767 : 15 |********** | |
| 27 | 32768 -> 65535 : 2 |* | |
| 28 | 65536 -> 131071 : 2 |* | |
| 29 | |
| 30 | The latency of the disk I/O is measured from the issue to the device to its |
| 31 | completion. A -Q option can be used to include time queued in the kernel. |
| 32 | |
| 33 | This example output shows a large mode of latency from about 128 microseconds |
| 34 | to about 32767 microseconds (33 milliseconds). The bulk of the I/O was |
| 35 | between 1 and 8 ms, which is the expected block device latency for |
| 36 | rotational storage devices. |
| 37 | |
| 38 | The highest latency seen while tracing was between 65 and 131 milliseconds: |
| 39 | the last row printed, for which there were 2 I/O. |
| 40 | |
| 41 | For efficiency, biolatency uses an in-kernel eBPF map to store timestamps |
| 42 | with requests, and another in-kernel map to store the histogram (the "count") |
| 43 | column, which is copied to user-space only when output is printed. These |
| Alex Bagehot | 3b9679a | 2016-02-06 16:01:02 +0000 | [diff] [blame] | 44 | methods lower the performance overhead when tracing is performed. |
| Brendan Gregg | 9fa1562 | 2015-09-21 15:51:11 -0700 | [diff] [blame] | 45 | |
| 46 | |
| 47 | In the following example, the -m option is used to print a histogram using |
| 48 | milliseconds as the units (which eliminates the first several rows), -T to |
| 49 | print timestamps with the output, and to print 1 second summaries 5 times: |
| 50 | |
| 51 | # ./biolatency -mT 1 5 |
| 52 | Tracing block device I/O... Hit Ctrl-C to end. |
| 53 | |
| 54 | 06:20:16 |
| 55 | msecs : count distribution |
| 56 | 0 -> 1 : 36 |**************************************| |
| 57 | 2 -> 3 : 1 |* | |
| 58 | 4 -> 7 : 3 |*** | |
| 59 | 8 -> 15 : 17 |***************** | |
| 60 | 16 -> 31 : 33 |********************************** | |
| 61 | 32 -> 63 : 7 |******* | |
| 62 | 64 -> 127 : 6 |****** | |
| 63 | |
| 64 | 06:20:17 |
| 65 | msecs : count distribution |
| 66 | 0 -> 1 : 96 |************************************ | |
| 67 | 2 -> 3 : 25 |********* | |
| 68 | 4 -> 7 : 29 |*********** | |
| 69 | 8 -> 15 : 62 |*********************** | |
| 70 | 16 -> 31 : 100 |**************************************| |
| 71 | 32 -> 63 : 62 |*********************** | |
| 72 | 64 -> 127 : 18 |****** | |
| 73 | |
| 74 | 06:20:18 |
| 75 | msecs : count distribution |
| 76 | 0 -> 1 : 68 |************************* | |
| 77 | 2 -> 3 : 76 |**************************** | |
| 78 | 4 -> 7 : 20 |******* | |
| 79 | 8 -> 15 : 48 |***************** | |
| 80 | 16 -> 31 : 103 |**************************************| |
| 81 | 32 -> 63 : 49 |****************** | |
| 82 | 64 -> 127 : 17 |****** | |
| 83 | |
| 84 | 06:20:19 |
| 85 | msecs : count distribution |
| 86 | 0 -> 1 : 522 |*************************************+| |
| 87 | 2 -> 3 : 225 |**************** | |
| 88 | 4 -> 7 : 38 |** | |
| 89 | 8 -> 15 : 8 | | |
| 90 | 16 -> 31 : 1 | | |
| 91 | |
| 92 | 06:20:20 |
| 93 | msecs : count distribution |
| 94 | 0 -> 1 : 436 |**************************************| |
| 95 | 2 -> 3 : 106 |********* | |
| 96 | 4 -> 7 : 34 |** | |
| 97 | 8 -> 15 : 19 |* | |
| 98 | 16 -> 31 : 1 | | |
| 99 | |
| 100 | How the I/O latency distribution changes over time can be seen. |
| 101 | |
| 102 | |
| 103 | |
| 104 | The -Q option begins measuring I/O latency from when the request was first |
| 105 | queued in the kernel, and includes queuing latency: |
| 106 | |
| 107 | # ./biolatency -Q |
| 108 | Tracing block device I/O... Hit Ctrl-C to end. |
| 109 | ^C |
| 110 | usecs : count distribution |
| 111 | 0 -> 1 : 0 | | |
| 112 | 2 -> 3 : 0 | | |
| 113 | 4 -> 7 : 0 | | |
| 114 | 8 -> 15 : 0 | | |
| 115 | 16 -> 31 : 0 | | |
| 116 | 32 -> 63 : 0 | | |
| 117 | 64 -> 127 : 0 | | |
| 118 | 128 -> 255 : 3 |* | |
| 119 | 256 -> 511 : 37 |************** | |
| 120 | 512 -> 1023 : 30 |*********** | |
| 121 | 1024 -> 2047 : 18 |******* | |
| 122 | 2048 -> 4095 : 22 |******** | |
| 123 | 4096 -> 8191 : 14 |***** | |
| 124 | 8192 -> 16383 : 48 |******************* | |
| 125 | 16384 -> 32767 : 96 |**************************************| |
| 126 | 32768 -> 65535 : 31 |************ | |
| 127 | 65536 -> 131071 : 26 |********** | |
| 128 | 131072 -> 262143 : 12 |**** | |
| 129 | |
| 130 | This better reflects the latency suffered by the application (if it is |
| 131 | synchronous I/O), whereas the default mode without kernel queueing better |
| 132 | reflects the performance of the device. |
| 133 | |
| 134 | Note that the storage device (and storage device controller) usually have |
| 135 | queues of their own, which are always included in the latency, with or |
| 136 | without -Q. |
| 137 | |
| 138 | |
| Brendan Gregg | 44e6877 | 2015-09-25 13:20:16 -0700 | [diff] [blame] | 139 | The -D option will print a histogram per disk. Eg: |
| 140 | |
| 141 | # ./biolatency -D |
| 142 | Tracing block device I/O... Hit Ctrl-C to end. |
| 143 | ^C |
| 144 | |
| 145 | Bucket disk = 'xvdb' |
| 146 | usecs : count distribution |
| 147 | 0 -> 1 : 0 | | |
| 148 | 2 -> 3 : 0 | | |
| 149 | 4 -> 7 : 0 | | |
| 150 | 8 -> 15 : 0 | | |
| 151 | 16 -> 31 : 0 | | |
| 152 | 32 -> 63 : 0 | | |
| 153 | 64 -> 127 : 0 | | |
| 154 | 128 -> 255 : 1 | | |
| 155 | 256 -> 511 : 33 |********************** | |
| 156 | 512 -> 1023 : 36 |************************ | |
| 157 | 1024 -> 2047 : 58 |****************************************| |
| 158 | 2048 -> 4095 : 51 |*********************************** | |
| 159 | 4096 -> 8191 : 21 |************** | |
| 160 | 8192 -> 16383 : 2 |* | |
| 161 | |
| 162 | Bucket disk = 'xvdc' |
| 163 | usecs : count distribution |
| 164 | 0 -> 1 : 0 | | |
| 165 | 2 -> 3 : 0 | | |
| 166 | 4 -> 7 : 0 | | |
| 167 | 8 -> 15 : 0 | | |
| 168 | 16 -> 31 : 0 | | |
| 169 | 32 -> 63 : 0 | | |
| 170 | 64 -> 127 : 0 | | |
| 171 | 128 -> 255 : 1 | | |
| 172 | 256 -> 511 : 38 |*********************** | |
| 173 | 512 -> 1023 : 42 |************************* | |
| 174 | 1024 -> 2047 : 66 |****************************************| |
| 175 | 2048 -> 4095 : 40 |************************ | |
| 176 | 4096 -> 8191 : 14 |******** | |
| 177 | |
| 178 | Bucket disk = 'xvda1' |
| 179 | usecs : count distribution |
| 180 | 0 -> 1 : 0 | | |
| 181 | 2 -> 3 : 0 | | |
| 182 | 4 -> 7 : 0 | | |
| 183 | 8 -> 15 : 0 | | |
| 184 | 16 -> 31 : 0 | | |
| 185 | 32 -> 63 : 0 | | |
| 186 | 64 -> 127 : 0 | | |
| 187 | 128 -> 255 : 0 | | |
| 188 | 256 -> 511 : 18 |********** | |
| 189 | 512 -> 1023 : 67 |************************************* | |
| 190 | 1024 -> 2047 : 35 |******************* | |
| 191 | 2048 -> 4095 : 71 |****************************************| |
| 192 | 4096 -> 8191 : 65 |************************************ | |
| 193 | 8192 -> 16383 : 65 |************************************ | |
| 194 | 16384 -> 32767 : 20 |*********** | |
| 195 | 32768 -> 65535 : 7 |*** | |
| 196 | |
| 197 | This output sows that xvda1 has much higher latency, usually between 0.5 ms |
| 198 | and 32 ms, whereas xvdc is usually between 0.2 ms and 4 ms. |
| 199 | |
| 200 | |
| Brendan Gregg | 9fa1562 | 2015-09-21 15:51:11 -0700 | [diff] [blame] | 201 | USAGE message: |
| 202 | |
| 203 | # ./biolatency -h |
| Brendan Gregg | 44e6877 | 2015-09-25 13:20:16 -0700 | [diff] [blame] | 204 | usage: biolatency [-h] [-T] [-Q] [-m] [-D] [interval] [count] |
| Brendan Gregg | 9fa1562 | 2015-09-21 15:51:11 -0700 | [diff] [blame] | 205 | |
| 206 | Summarize block device I/O latency as a histogram |
| 207 | |
| 208 | positional arguments: |
| 209 | interval output interval, in seconds |
| 210 | count number of outputs |
| 211 | |
| 212 | optional arguments: |
| 213 | -h, --help show this help message and exit |
| 214 | -T, --timestamp include timestamp on output |
| 215 | -Q, --queued include OS queued time in I/O time |
| 216 | -m, --milliseconds millisecond histogram |
| Brendan Gregg | 44e6877 | 2015-09-25 13:20:16 -0700 | [diff] [blame] | 217 | -D, --disks print a histogram per disk device |
| Brendan Gregg | 9fa1562 | 2015-09-21 15:51:11 -0700 | [diff] [blame] | 218 | |
| 219 | examples: |
| 220 | ./biolatency # summarize block I/O latency as a histogram |
| 221 | ./biolatency 1 10 # print 1 second summaries, 10 times |
| 222 | ./biolatency -mT 1 # 1s summaries, milliseconds, and timestamps |
| 223 | ./biolatency -Q # include OS queued time in I/O time |
| Brendan Gregg | 44e6877 | 2015-09-25 13:20:16 -0700 | [diff] [blame] | 224 | ./biolatency -D # show each disk device separately |