iolatency_example.txt

Demonstrations of iolatency, the Linux ftrace version.


Here's a busy system doing over 4k disk IOPS:

# ./iolatency
Tracing block I/O. Output every 1 seconds. Ctrl-C to end.

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 4381     |######################################|
       1 -> 2       : 9        |#                                     |
       2 -> 4       : 5        |#                                     |
       4 -> 8       : 0        |                                      |
       8 -> 16      : 1        |#                                     |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 4053     |######################################|
       1 -> 2       : 18       |#                                     |
       2 -> 4       : 9        |#                                     |
       4 -> 8       : 2        |#                                     |
       8 -> 16      : 1        |#                                     |
      16 -> 32      : 1        |#                                     |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 4658     |######################################|
       1 -> 2       : 9        |#                                     |
       2 -> 4       : 2        |#                                     |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 4298     |######################################|
       1 -> 2       : 17       |#                                     |
       2 -> 4       : 10       |#                                     |
       4 -> 8       : 1        |#                                     |
       8 -> 16      : 1        |#                                     |
^C
Ending tracing...

Disk I/O latency is usually between 0 and 1 milliseconds, as this system uses
SSDs. There are occasional outliers, up to the 16->32 ms range.

Identifying outliers like these is difficult from iostat(1) alone, which at
the same time reported:

# iostat 1
[...]
avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.53    0.00    1.05   46.84    0.53   51.05

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
xvdap1            0.00     0.00    0.00   28.00     0.00   112.00     8.00     0.02    0.71    0.00    0.71   0.29   0.80
xvdb              0.00     0.00 2134.00    0.00 18768.00     0.00    17.59     0.51    0.24    0.24    0.00   0.23  50.00
xvdc              0.00     0.00 2088.00    0.00 18504.00     0.00    17.72     0.47    0.22    0.22    0.00   0.22  46.40
md0               0.00     0.00 4222.00    0.00 37256.00     0.00    17.65     0.00    0.00    0.00    0.00   0.00   0.00

I/O latency ("await") averages 0.24 and 0.22 ms for our busy disks, but this
output doesn't show that occasionally is much higher.

To get more information on these I/O, try the iosnoop(8) tool.


The -Q option includes the block I/O queued time, by tracing based on
block_rq_insert instead of block_rq_issue:

# ./iolatency -Q
Tracing block I/O. Output every 1 seconds. Ctrl-C to end.

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 1913     |######################################|
       1 -> 2       : 438      |#########                             |
       2 -> 4       : 100      |##                                    |
       4 -> 8       : 145      |###                                   |
       8 -> 16      : 43       |#                                     |
      16 -> 32      : 43       |#                                     |
      32 -> 64      : 1        |#                                     |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 2360     |######################################|
       1 -> 2       : 132      |###                                   |
       2 -> 4       : 72       |##                                    |
       4 -> 8       : 14       |#                                     |
       8 -> 16      : 1        |#                                     |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 2138     |######################################|
       1 -> 2       : 496      |#########                             |
       2 -> 4       : 81       |##                                    |
       4 -> 8       : 40       |#                                     |
       8 -> 16      : 1        |#                                     |
      16 -> 32      : 2        |#                                     |
^C
Ending tracing...

I use this along with the default mode to identify problems of load (queueing)
vs problems of the device, which is shown by default.


Here's a more interesting system. This is doing a mixed read/write workload,
and has a pretty awful latency distribution:

# ./iolatency 5 3
Tracing block I/O. Output every 5 seconds.

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 2809     |######################################|
       1 -> 2       : 32       |#                                     |
       2 -> 4       : 14       |#                                     |
       4 -> 8       : 6        |#                                     |
       8 -> 16      : 7        |#                                     |
      16 -> 32      : 14       |#                                     |
      32 -> 64      : 39       |#                                     |
      64 -> 128     : 1556     |######################                |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 3027     |######################################|
       1 -> 2       : 19       |#                                     |
       2 -> 4       : 6        |#                                     |
       4 -> 8       : 5        |#                                     |
       8 -> 16      : 3        |#                                     |
      16 -> 32      : 7        |#                                     |
      32 -> 64      : 14       |#                                     |
      64 -> 128     : 540      |#######                               |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 2939     |######################################|
       1 -> 2       : 25       |#                                     |
       2 -> 4       : 15       |#                                     |
       4 -> 8       : 2        |#                                     |
       8 -> 16      : 3        |#                                     |
      16 -> 32      : 7        |#                                     |
      32 -> 64      : 17       |#                                     |
      64 -> 128     : 936      |#############                         |

Ending tracing...

It's multi-modal, with most I/O taking 0 to 1 milliseconds, then many between
64 and 128 milliseconds. This is how it looks in iostat:

# iostat -x 1

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.52    0.00   12.37   32.99    0.00   54.12

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
xvdap1            0.00    12.00    0.00  156.00     0.00 19968.00   256.00    52.17  184.38    0.00  184.38   2.33  36.40
xvdb              0.00     0.00  298.00    0.00  2732.00     0.00    18.34     0.04    0.12    0.12    0.00   0.11   3.20
xvdc              0.00     0.00  297.00    0.00  2712.00     0.00    18.26     0.08    0.27    0.27    0.00   0.24   7.20
md0               0.00     0.00  595.00    0.00  5444.00     0.00    18.30     0.00    0.00    0.00    0.00   0.00   0.00

Fortunately, it turns out that the high latency is to xvdap1, which is for files
from a low priority application (processing and writing log files). A high
priority application is reading from the other disks, xvdb and xvdc.

Examining xvdap1 only:

# ./iolatency -d 202,1 5
Tracing block I/O. Output every 5 seconds. Ctrl-C to end.

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 38       |##                                    |
       1 -> 2       : 18       |#                                     |
       2 -> 4       : 0        |                                      |
       4 -> 8       : 0        |                                      |
       8 -> 16      : 5        |#                                     |
      16 -> 32      : 11       |#                                     |
      32 -> 64      : 26       |##                                    |
      64 -> 128     : 894      |######################################|

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 75       |###                                   |
       1 -> 2       : 11       |#                                     |
       2 -> 4       : 0        |                                      |
       4 -> 8       : 4        |#                                     |
       8 -> 16      : 4        |#                                     |
      16 -> 32      : 7        |#                                     |
      32 -> 64      : 13       |#                                     |
      64 -> 128     : 1141     |######################################|

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 61       |########                              |
       1 -> 2       : 21       |###                                   |
       2 -> 4       : 5        |#                                     |
       4 -> 8       : 1        |#                                     |
       8 -> 16      : 5        |#                                     |
      16 -> 32      : 7        |#                                     |
      32 -> 64      : 19       |###                                   |
      64 -> 128     : 324      |######################################|
     128 -> 256     : 7        |#                                     |
     256 -> 512     : 26       |####                                  |
^C
Ending tracing...

And now xvdb:

# ./iolatency -d 202,16 5
Tracing block I/O. Output every 5 seconds. Ctrl-C to end.

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 1427     |######################################|
       1 -> 2       : 5        |#                                     |
       2 -> 4       : 3        |#                                     |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 1409     |######################################|
       1 -> 2       : 6        |#                                     |
       2 -> 4       : 1        |#                                     |
       4 -> 8       : 1        |#                                     |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 1478     |######################################|
       1 -> 2       : 6        |#                                     |
       2 -> 4       : 5        |#                                     |
       4 -> 8       : 0        |                                      |
       8 -> 16      : 2        |#                                     |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 1437     |######################################|
       1 -> 2       : 5        |#                                     |
       2 -> 4       : 7        |#                                     |
       4 -> 8       : 0        |                                      |
       8 -> 16      : 1        |#                                     |
[...]

While that's much better, it is reaching the 8 - 16 millisecond range,
and these are SSDs with a light workload (~1500 IOPS).

I already know from iosnoop(8) analysis the reason for these high latency
outliers: they are queued behind writes. However, these writes are to a
different disk -- somewhere in this virtualized guest (Xen) there may be a
shared I/O queue.

One way to explore this is to reduce the queue length for the low priority disk,
so that it is less likely to pollute any shared queue. (There are other ways to
investigate and fix this too.) Here I reduce the disk queue length from its
default of 128 to 4:

# echo 4 > /sys/block/xvda1/queue/nr_requests

The overall distribution looks much better:

# ./iolatency 5
Tracing block I/O. Output every 5 seconds. Ctrl-C to end.

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 3005     |######################################|
       1 -> 2       : 19       |#                                     |
       2 -> 4       : 9        |#                                     |
       4 -> 8       : 45       |#                                     |
       8 -> 16      : 859      |###########                           |
      16 -> 32      : 16       |#                                     |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 2959     |######################################|
       1 -> 2       : 43       |#                                     |
       2 -> 4       : 16       |#                                     |
       4 -> 8       : 39       |#                                     |
       8 -> 16      : 1009     |#############                         |
      16 -> 32      : 76       |#                                     |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 3031     |######################################|
       1 -> 2       : 27       |#                                     |
       2 -> 4       : 9        |#                                     |
       4 -> 8       : 24       |#                                     |
       8 -> 16      : 422      |######                                |
      16 -> 32      : 5        |#                                     |
^C
Ending tracing...

Latency only reaching 32 ms.

Our important disk didn't appear to change much -- maybe a slight improvement
to the outliers:

# ./iolatency -d 202,16 5
Tracing block I/O. Output every 5 seconds. Ctrl-C to end.

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 1449     |######################################|
       1 -> 2       : 6        |#                                     |
       2 -> 4       : 5        |#                                     |
       4 -> 8       : 1        |#                                     |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 1519     |######################################|
       1 -> 2       : 12       |#                                     |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 1466     |######################################|
       1 -> 2       : 2        |#                                     |
       2 -> 4       : 3        |#                                     |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 1460     |######################################|
       1 -> 2       : 4        |#                                     |
       2 -> 4       : 7        |#                                     |
[...]

And here's the other disk after the queue length change:

# ./iolatency -d 202,1 5
Tracing block I/O. Output every 5 seconds. Ctrl-C to end.

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 85       |###                                   |
       1 -> 2       : 12       |#                                     |
       2 -> 4       : 21       |#                                     |
       4 -> 8       : 76       |##                                    |
       8 -> 16      : 1539     |######################################|
      16 -> 32      : 10       |#                                     |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 123      |##################                    |
       1 -> 2       : 8        |##                                    |
       2 -> 4       : 6        |#                                     |
       4 -> 8       : 17       |###                                   |
       8 -> 16      : 270      |######################################|
      16 -> 32      : 2        |#                                     |

  >=(ms) .. <(ms)   : I/O      |Distribution                          |
       0 -> 1       : 91       |###                                   |
       1 -> 2       : 23       |#                                     |
       2 -> 4       : 8        |#                                     |
       4 -> 8       : 71       |###                                   |
       8 -> 16      : 1223     |######################################|
      16 -> 32      : 12       |#                                     |
^C
Ending tracing...

Much better looking distribution.


Use -h to print the USAGE message:

# ./iolatency -h
USAGE: iolatency [-hQT] [-d device] [-i iotype] [interval [count]]
                 -d device       # device string (eg, "202,1)
                 -i iotype       # match type (eg, '*R*' for all reads)
                 -Q              # use queue insert as start time
                 -T              # timestamp on output
                 -h              # this usage message
                 interval        # summary interval, seconds (default 1)
                 count           # number of summaries
  eg,
       iolatency                 # summarize latency every second
       iolatency -Q              # include block I/O queue time
       iolatency 5 2             # 2 x 5 second summaries
       iolatency -i '*R*'        # trace reads
       iolatency -d 202,1        # trace device 202,1 only

See the man page and example file for more info.