**************************************************************************
* The following are additional notes on ALL of the *snoop programs (such as
* execsnoop, iosnoop, ..., and dapptrace, dtruss).
*
* $Id: ALLsnoop_notes.txt 44 2007-09-17 07:47:20Z brendan $
*
* COPYRIGHT: Copyright (c) 2007 Brendan Gregg.
**************************************************************************
* The output seems shuffled?
Beware - due to the (current) way DTrace works, on multi-CPU systems there
is no guarentee that if you print traced events the output is in the same
order that the events occured.
This is because events details are placed in kernel per-CPU buffers, and then
dumped in sequence by the DTrace consumer (/usr/sbin/dtrace) whenever it
wakes up ("switchrate" tunable). The DTrace consumer reads and prints the
buffers one by one, it doesn't combine them and sort them.
To demonstrate this,
# dtrace -n 'profile:::profile-3hz { trace(timestamp); }'
dtrace: description 'profile-3hz ' matched 1 probe
CPU ID FUNCTION:NAME
0 41241 :profile-3hz 1898015274778547
0 41241 :profile-3hz 1898015608118262
0 41241 :profile-3hz 1898015941430060
1 41241 :profile-3hz 1898015275499014
1 41241 :profile-3hz 1898015609173485
1 41241 :profile-3hz 1898015942505828
2 41241 :profile-3hz 1898015275351257
2 41241 :profile-3hz 1898015609180861
2 41241 :profile-3hz 1898015942512708
3 41241 :profile-3hz 1898015274803528
3 41241 :profile-3hz 1898015608120522
3 41241 :profile-3hz 1898015941449884
^C
If you read the timestamps carefully, you'll see that they aren't quite
in chronological order. If you look at the CPU column while reading the
timestamps, the way DTrace works should become clear.
Most of the snoop tools have a switchrate of 10hz, so events may be shuffled
within a tenth of a second - not hugely noticable.
This isn't really a problem anyway. If you must have the output in the correct
order, find the switch that prints timestamps and then sort the output.
As an example,
# iosnoop -t > out.iosnoop
^C
# sort -n out.iosnoop
TIME UID PID D BLOCK SIZE COMM PATHNAME
183710958520 0 3058 W 10507848 4096 sync /var/log/pool/poold
183710990358 0 3058 W 6584858 1024 sync /etc/motd
183711013469 0 3058 W 60655 9216 sync <none>
183711020149 0 3058 W 60673 1024 sync <none>
All shell-wrapped scripts should have some way to print timestamps, and
many DTrace-only scripts print timestamps by default. If you find a script
that doesn't print timestamps, it should be trivial for you to add an
extra column.
To add a microsecond-since-boot time column to a script, try adding this
before every printf() you find,
printf("%-16d ", timestamp / 1000);
except for the header line, where you can add this,
printf("%-16s ", "TIME(us)");
Now you will be able to post sort the script output on the TIME(us) column.
In practise, I find post sorting the output a little annoying at times,
and use a couple of other ways to prevent shuffling from happening in the
first place:
- offline all CPUs but one when running flow scripts. Naturally, you
probably don't want to do this on production servers, this is a trick
that may be handy for when developing on workstations or laptops. Bear
in mind that if you are trying to DTrace certain issues, such as
multi-thread locking contention, then offlining most CPUs may eliminate
the issue you are trying to observe.
- pbind the target process of interest to a single CPU. Most OSes provide
a way to glue a process to a single CPU; Solaris has both pbind and psrset.
Another way to solve this problem would be to enhance DTrace to always print
in-order output. Maybe this will be done one day; maybe by the time you
are reading this it has already been done?
