svn commit: r322863 - head/sys/cam

[Fabian Keil]

Warner Losh <imp at FreeBSD.org> wrote:

> Author: imp
> Date: Thu Aug 24 22:11:10 2017
> New Revision: 322863
> URL: https://svnweb.freebsd.org/changeset/base/322863
> 
> Log:
>   Expand the latency tracking array from 1.024s to 8.192s to help track
>   extreme outliers from dodgy drives. Adjust comments to reflect this,
>   and make sure that the number of latency buckets match in the two
>   places where it matters.

It also matters for the sysctl description in cam_iosched_iop_stats_sysctl_init().

Would you mind increasing LAT_BUCKETS a bit more? I've recently set it to
17 in ElectroBSD as writes to my laptop's disk occasionally take a while
under load:

fk at t520 ~ $sudo /usr/src/share/dtrace/bio-request-latency
[...]
  ada0                                                BIO_WRITE
           value  ------------- Distribution ------------- count
              16 |                                         0
              32 |                                         1
              64 |                                         34
             128 |@                                        582
             256 |@@@@@@@@@@                               10608
             512 |@@@@@                                    5574
            1024 |@@                                       2454
            2048 |@@@@                                     3872
            4096 |@@@@                                     4043
            8192 |@@@@                                     4115
           16384 |@@@@@                                    4953
           32768 |@@@                                      3656
           65536 |@@                                       2252
          131072 |@                                        764
          262144 |                                         331
          524288 |                                         291
         1048576 |                                         53
         2097152 |                                         9
         4194304 |                                         4
         8388608 |                                         6
        16777216 |                                         2
        33554432 |                                         0

  ada0                                                BIO_READ
           value  ------------- Distribution ------------- count
              16 |                                         0
              32 |                                         1
              64 |                                         126
             128 |@                                        1205
             256 |@@@@@                                    6359
             512 |@@@                                      3987
            1024 |@@                                       2044
            2048 |@                                        1384
            4096 |@@                                       2853
            8192 |@@@@@                                    6012
           16384 |@@@@@@                                   7254
           32768 |@@@@@                                    6524
           65536 |@@@@                                     4945
          131072 |@@@                                      3006
          262144 |@                                        1194
          524288 |@                                        620
         1048576 |                                         162
         2097152 |                                         0

Disk     Bio Type       Requests |                 Latency in us           |   iops   Throughput
                                 |       avg    stddev       max       min |
ada0     BIO_FLUSH           253 |    145927    162796   1253731     10293 |      0        0 k/s
ada0     BIO_WRITE         43604 |     30345    261090  30069820        61 |     43     1994 k/s
ada0     BIO_READ          47676 |     56039    128083   1691813        63 |     47     1260 k/s

While proper tuning helps, the latency stats are also useful on
systems where the iosched limiters haven't been enabled (yet).

Fabian
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