Wednesday, May 29, 2013

Dtrace training -- its worth it!

While its fresh in my mind, I wanted to cover the wondering Dtrace training I had with Max Bruning and Brendan Gregg. The three day course is occasionally offered by Joyent, and here is an example synopsis. Not only do you get the meaty book on the subject, but the workbook and the labs you do are challenging. Sadly I came in a not too distant second on the 'ole leader board for solving the most labs.

What is more interesting was how quickly I found myself using the tool after the class. Although I heavily utilize KVM instances in Joyent's SDC, I got a request the following day as students were approaching "tape out" for their chips. Various students weren't cleaning up after themselves, with simulations running endlessly. The usual quip that "the internet is slow" or some such came quickly, in this case the statement that the network to and from the primary VM for some Ansys tools was very slow. Looking inside this 16 core, 64GB VM running Linux, it was not entirely apparent what was going wrong. The load was high, and in the end, I could have done some basic thread counting and discovered the answer if I knew where to look. However, I was able to use Dtrace in an opaque way and see from the hardware node that the qemu/kvm process was using its 16 cpu threads full tilt, and it was not I/O bound. Looking back into the Linux VM, it took little time to find that it was beyond the 16 CPU cores, trying to run at least 17 full time tasks. One task had over 20GB of memory mapped, and each time it yielded execution time and reacquired its CPU, it would undoubtedly spend extra effort in dealing with its large memory payload.

Killing off just one process here resolved the performance problem. VMs are sort of the worse case scenario for Dtrace, but it still guided me to the solution. The trick with the book is that one needs real world examples and enough practice to get the specific predicates and formation of queries down. Once you are over the hump, and I'm not sure I'm there yet, you'll find Dtrace to be indispensable.

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