************************************************************************** * 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 183711020149 0 3058 W 60673 1024 sync 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?