| Demonstrations of biolatency, the Linux eBPF/bcc version. |
| |
| |
| biolatency traces block device I/O (disk I/O), and records the distribution |
| of I/O latency (time), printing this as a histogram when Ctrl-C is hit. |
| For example: |
| |
| # ./biolatency |
| Tracing block device I/O... Hit Ctrl-C to end. |
| ^C |
| usecs : count distribution |
| 0 -> 1 : 0 | | |
| 2 -> 3 : 0 | | |
| 4 -> 7 : 0 | | |
| 8 -> 15 : 0 | | |
| 16 -> 31 : 0 | | |
| 32 -> 63 : 0 | | |
| 64 -> 127 : 1 | | |
| 128 -> 255 : 12 |******** | |
| 256 -> 511 : 15 |********** | |
| 512 -> 1023 : 43 |******************************* | |
| 1024 -> 2047 : 52 |**************************************| |
| 2048 -> 4095 : 47 |********************************** | |
| 4096 -> 8191 : 52 |**************************************| |
| 8192 -> 16383 : 36 |************************** | |
| 16384 -> 32767 : 15 |********** | |
| 32768 -> 65535 : 2 |* | |
| 65536 -> 131071 : 2 |* | |
| |
| The latency of the disk I/O is measured from the issue to the device to its |
| completion. A -Q option can be used to include time queued in the kernel. |
| |
| This example output shows a large mode of latency from about 128 microseconds |
| to about 32767 microseconds (33 milliseconds). The bulk of the I/O was |
| between 1 and 8 ms, which is the expected block device latency for |
| rotational storage devices. |
| |
| The highest latency seen while tracing was between 65 and 131 milliseconds: |
| the last row printed, for which there were 2 I/O. |
| |
| For efficiency, biolatency uses an in-kernel eBPF map to store timestamps |
| with requests, and another in-kernel map to store the histogram (the "count") |
| column, which is copied to user-space only when output is printed. These |
| methods lower the performance overhead when tracing is performed. |
| |
| |
| In the following example, the -m option is used to print a histogram using |
| milliseconds as the units (which eliminates the first several rows), -T to |
| print timestamps with the output, and to print 1 second summaries 5 times: |
| |
| # ./biolatency -mT 1 5 |
| Tracing block device I/O... Hit Ctrl-C to end. |
| |
| 06:20:16 |
| msecs : count distribution |
| 0 -> 1 : 36 |**************************************| |
| 2 -> 3 : 1 |* | |
| 4 -> 7 : 3 |*** | |
| 8 -> 15 : 17 |***************** | |
| 16 -> 31 : 33 |********************************** | |
| 32 -> 63 : 7 |******* | |
| 64 -> 127 : 6 |****** | |
| |
| 06:20:17 |
| msecs : count distribution |
| 0 -> 1 : 96 |************************************ | |
| 2 -> 3 : 25 |********* | |
| 4 -> 7 : 29 |*********** | |
| 8 -> 15 : 62 |*********************** | |
| 16 -> 31 : 100 |**************************************| |
| 32 -> 63 : 62 |*********************** | |
| 64 -> 127 : 18 |****** | |
| |
| 06:20:18 |
| msecs : count distribution |
| 0 -> 1 : 68 |************************* | |
| 2 -> 3 : 76 |**************************** | |
| 4 -> 7 : 20 |******* | |
| 8 -> 15 : 48 |***************** | |
| 16 -> 31 : 103 |**************************************| |
| 32 -> 63 : 49 |****************** | |
| 64 -> 127 : 17 |****** | |
| |
| 06:20:19 |
| msecs : count distribution |
| 0 -> 1 : 522 |*************************************+| |
| 2 -> 3 : 225 |**************** | |
| 4 -> 7 : 38 |** | |
| 8 -> 15 : 8 | | |
| 16 -> 31 : 1 | | |
| |
| 06:20:20 |
| msecs : count distribution |
| 0 -> 1 : 436 |**************************************| |
| 2 -> 3 : 106 |********* | |
| 4 -> 7 : 34 |** | |
| 8 -> 15 : 19 |* | |
| 16 -> 31 : 1 | | |
| |
| How the I/O latency distribution changes over time can be seen. |
| |
| |
| |
| The -Q option begins measuring I/O latency from when the request was first |
| queued in the kernel, and includes queuing latency: |
| |
| # ./biolatency -Q |
| Tracing block device I/O... Hit Ctrl-C to end. |
| ^C |
| usecs : count distribution |
| 0 -> 1 : 0 | | |
| 2 -> 3 : 0 | | |
| 4 -> 7 : 0 | | |
| 8 -> 15 : 0 | | |
| 16 -> 31 : 0 | | |
| 32 -> 63 : 0 | | |
| 64 -> 127 : 0 | | |
| 128 -> 255 : 3 |* | |
| 256 -> 511 : 37 |************** | |
| 512 -> 1023 : 30 |*********** | |
| 1024 -> 2047 : 18 |******* | |
| 2048 -> 4095 : 22 |******** | |
| 4096 -> 8191 : 14 |***** | |
| 8192 -> 16383 : 48 |******************* | |
| 16384 -> 32767 : 96 |**************************************| |
| 32768 -> 65535 : 31 |************ | |
| 65536 -> 131071 : 26 |********** | |
| 131072 -> 262143 : 12 |**** | |
| |
| This better reflects the latency suffered by the application (if it is |
| synchronous I/O), whereas the default mode without kernel queueing better |
| reflects the performance of the device. |
| |
| Note that the storage device (and storage device controller) usually have |
| queues of their own, which are always included in the latency, with or |
| without -Q. |
| |
| |
| The -D option will print a histogram per disk. Eg: |
| |
| # ./biolatency -D |
| Tracing block device I/O... Hit Ctrl-C to end. |
| ^C |
| |
| Bucket disk = 'xvdb' |
| usecs : count distribution |
| 0 -> 1 : 0 | | |
| 2 -> 3 : 0 | | |
| 4 -> 7 : 0 | | |
| 8 -> 15 : 0 | | |
| 16 -> 31 : 0 | | |
| 32 -> 63 : 0 | | |
| 64 -> 127 : 0 | | |
| 128 -> 255 : 1 | | |
| 256 -> 511 : 33 |********************** | |
| 512 -> 1023 : 36 |************************ | |
| 1024 -> 2047 : 58 |****************************************| |
| 2048 -> 4095 : 51 |*********************************** | |
| 4096 -> 8191 : 21 |************** | |
| 8192 -> 16383 : 2 |* | |
| |
| Bucket disk = 'xvdc' |
| usecs : count distribution |
| 0 -> 1 : 0 | | |
| 2 -> 3 : 0 | | |
| 4 -> 7 : 0 | | |
| 8 -> 15 : 0 | | |
| 16 -> 31 : 0 | | |
| 32 -> 63 : 0 | | |
| 64 -> 127 : 0 | | |
| 128 -> 255 : 1 | | |
| 256 -> 511 : 38 |*********************** | |
| 512 -> 1023 : 42 |************************* | |
| 1024 -> 2047 : 66 |****************************************| |
| 2048 -> 4095 : 40 |************************ | |
| 4096 -> 8191 : 14 |******** | |
| |
| Bucket disk = 'xvda1' |
| usecs : count distribution |
| 0 -> 1 : 0 | | |
| 2 -> 3 : 0 | | |
| 4 -> 7 : 0 | | |
| 8 -> 15 : 0 | | |
| 16 -> 31 : 0 | | |
| 32 -> 63 : 0 | | |
| 64 -> 127 : 0 | | |
| 128 -> 255 : 0 | | |
| 256 -> 511 : 18 |********** | |
| 512 -> 1023 : 67 |************************************* | |
| 1024 -> 2047 : 35 |******************* | |
| 2048 -> 4095 : 71 |****************************************| |
| 4096 -> 8191 : 65 |************************************ | |
| 8192 -> 16383 : 65 |************************************ | |
| 16384 -> 32767 : 20 |*********** | |
| 32768 -> 65535 : 7 |*** | |
| |
| This output sows that xvda1 has much higher latency, usually between 0.5 ms |
| and 32 ms, whereas xvdc is usually between 0.2 ms and 4 ms. |
| |
| |
| USAGE message: |
| |
| # ./biolatency -h |
| usage: biolatency [-h] [-T] [-Q] [-m] [-D] [interval] [count] |
| |
| Summarize block device I/O latency as a histogram |
| |
| positional arguments: |
| interval output interval, in seconds |
| count number of outputs |
| |
| optional arguments: |
| -h, --help show this help message and exit |
| -T, --timestamp include timestamp on output |
| -Q, --queued include OS queued time in I/O time |
| -m, --milliseconds millisecond histogram |
| -D, --disks print a histogram per disk device |
| |
| examples: |
| ./biolatency # summarize block I/O latency as a histogram |
| ./biolatency 1 10 # print 1 second summaries, 10 times |
| ./biolatency -mT 1 # 1s summaries, milliseconds, and timestamps |
| ./biolatency -Q # include OS queued time in I/O time |
| ./biolatency -D # show each disk device separately |
