Tuesday, August 5, 2014

kstat changes in illumos

One of the nice changes to the kstat (kernel statistics) command in illumos is its conversion to C from perl. There were several areas in the illumos (nee OpenSolaris) code where perl had been used. But these were too few to maintain critical mass and it is difficult for interpreted runtimes to change at the pace of an OS, so keeping the two in lockstep is simply not worthwhile. So the few places where parts of illumos were written in perl have been replaced by native C implementations.

The kstat(1m) command rewritten in C was contributed by David Höppner, an active member of the illumos community. It is fast and efficient at filtering and printing kstats. By contrast, the old perl version had to start perl (an interpreter), find and load the kstat-to-perl module, and then filter and print the kstats. Internal to the kernel, kstats are stored as a name-value list (nvlist) containing strongly-typed data. Many of these are 64-bit integers. This poses a problem for the version of perl used (5.12) as the 64-bit support is dependent on the compiled version and illumos can be compiled for both 32 and 64 bit processors. To compensate for this mismatch, the following was added to the man page for kstat(3perl):

Several of the statistics provided by the kstat facility are stored as 64-bit integer values. Perl 5 does not yet internally support 64-bit integers, so these values are approximated in this module. There are two classes of 64-bit value to be dealt with: 64-bit intervals and times

These are the crtime and snaptime fields of all the statistics hashes, and the wtime, wlentime, wlastupdate, rtime, rlentime and rlastupdate fields of the kstat I/O statistics structures. These are measured by the kstat facility in nanoseconds, meaning that a 32-bit value would represent approximately 4 seconds. The alternative is to store the values as floating-point numbers, which offer approximately 53 bits of precision on present hardware. 64-bit intervals and timers as floating point values expressed in seconds, meaning that time-related kstats are being rounded to approximately microsecond resolution.

It is not useful to store these values as 32-bit values. As noted above, floating-point values offer 53 bitsof precision. Accordingly, all 64-bit counters are stored as floating-point values.

For consumers of the kstat(1m) command output, this means that kstat I/O statistics are stored as seconds (floating point) instead of nanoseconds. For example, with formatting adjusted for readability:
Perl-based kstat(1m)
# kstat -p sd:0:sd0
sd:0:sd0:class        disk
sd:0:sd0:crtime       1855326.99995062
sd:0:sd0:nread        380919301
sd:0:sd0:nwritten     1984175104
sd:0:sd0:rcnt         0
sd:0:sd0:reads        18455
sd:0:sd0:rlastupdate  2371703.49260763
sd:0:sd0:rlentime     147.154123471
sd:0:sd0:rtime        49.399890683
sd:0:sd0:snaptime     2371828.77138052
sd:0:sd0:wcnt         0
sd:0:sd0:wlastupdate  2371703.49174494
sd:0:sd0:wlentime     2.425675727
sd:0:sd0:writes       103602
sd:0:sd0:wtime        1.43643661

C-based kstat(1m)
# kstat -p sd:0:sd0
sd:0:sd0:class        disk
sd:0:sd0:crtime       244.271312204
sd:0:sd0:nread        25549493
sd:0:sd0:nwritten     1698218496
sd:0:sd0:rcnt         0
sd:0:sd0:reads        4043
sd:0:sd0:rlastupdate  104543293563241
sd:0:sd0:rlentime     68750036336
sd:0:sd0:rtime        64365048052
sd:0:sd0:snaptime     104509.092582653
sd:0:sd0:wcnt         0
sd:0:sd0:wlastupdate  104543293482995
sd:0:sd0:wlentime     4569934990
sd:0:sd0:writes       289766
sd:0:sd0:wtime        4551425719

I find kstat(1m) output to be very convenient for historical tracking and use it often. If you do too, then be aware of this conversion.

One of the best features of the new C-based kstat(1m) is the ability to get kstats as JSON. This is even more useful than the "parseable" output shown previously.
# kstat -j sd:0:sd0
  "module": "sd",
  "instance": 0,
  "name": "sd0",
  "class": "disk",
  "type": 3,
  "snaptime": 104547.013504692,
  "data": {
    "crtime": 244.271312204,
    "nread": 25549493,
    "nwritten": 1700980224,
    "rcnt": 0,
    "reads": 4043,
    "rlastupdate": 104733296813446,
    "rlentime": 68901598866,
    "rtime": 64513785819,
    "snaptime": 104547.013504692,
    "wcnt": 0,
    "wlastupdate": 104733296708770,
    "wlentime": 4579560895,
    "writes": 290404,
    "wtime": 4561051625

Using JSON has the added advantage of being easy to parse without making assumptions about the data. For example, did you know that some kernel modules use ':' in the kstat module, instance, name, or statistic? This makes using the parseable output a little bit tricky. The JSON output is not affected and is readily and consistently readable or storable in the many tools that support JSON.

Now you can see how to take advantage of the kstat(1m) command and how it has evolved under illumos to be more friendly to building tools and taking quick measurements. Go forth and kstat!

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