When I asked for tools to profile Rails apps, someone pointed at DTrace. Since I work on MacOSX stations and deploy on OpenSolaris, it is a valid way to go. But I have little knowledge of DTrace.
Besides the usual suspect, Sun DTrace page and the avaliable info there, is there any other killer pointer to learn Dtrace out there?
I found that reading dtrace version of the usual admin monitoring tools of Mac OS X was a great help truss -> dtruss. There are at least a dozen different scripts that you can read. (Most begining by "d".)
There also is /Developer/Applications/Instruments.app which is a graphical app which generates dtrace scripts.
Also, Wikipedia's DTrace page has loads of links which should help.
You can also try the "dtrace toolkit" it has scripts for lots of monitoring purposes to give you a kick start.
Also you can attach X-Code's "Instruments" to any process. After this gives you a general idea of your app you can give a try to write your own scripts
I did a 35 min video and showed some DTrace starter examples that you can find here: http://cocoasamurai.blogspot.com/2008/05/dtrace-for-cocoa-developers.html which were intended for introducing DTrace to Cocoa Developers, but it can work for anybody really
Watch the Brian Cantrill DTrace video. This is a great demo-based talk and Cantrill is one of the DTrace creators.
http://video.google.com/videoplay?docid=-8002801113289007228