You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
224 lines
12 KiB
224 lines
12 KiB
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
|
|
|