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Introduction to Intel Pin

This blogpost is an introduction to Intel’s Pin dynamic instrumentation framework. Pin and the pintools were brought to my attention by Mahmoud Hatem in his blogpost Tracing Memory access of an oracle process: Intel PinTools. The Pin framework provides an API that abstracts instruction-set specifics (on the CPU layer). Because this is a dynamic binary instrumentation tool, it requires no recompiling of source code. This means we can use it with programs like the Oracle database executable.
The Pin framework download comes with a set of pre-created tools called ‘Pintools’. Some of these tools are really useful for Oracle investigation and research.

Watching is in the eye of the beholder

Recently I was investigating the inner working of Oracle. One of the things that is fundamental to the Oracle database is the SCN (system change number). SCNs are used to synchronise changes in the database. There is one source for SCNs in every instance (kcbgscn; the global or current SCN in the fixed SGA), and there are multiple tasks for which Oracle keeps track of synchronisation using SCNs. A few of these tasks for which Oracle stores and uses SCNs to keep track of progress are on disk SCN and lwn SCN.

This blogpost is about some oddities I found when using gdb (the GNU debugger) to watch memory locations of a running Oracle database. This should not be done on a production instance, and is purely for research purposes. Only use the methods mentioned in this article if you are absolutely sure what you are doing, and/or if you are using an Oracle instance that can be crashed and can be restored.

Linux strace doesn’t lie after all.

strace is a linux utility to profile system calls. Using strace you can see the system calls that a process executes, in order to investigate the inner working or performance. In my presentation about multiblock reads I put the text ‘strace lies’. This is NOT correct. My current understanding is that strace does show every system call made by an executable. So…why did I make that statement? (editorial note: this article dives into the inner working of Linux AIO)