Sunday, June 3, 2012

##what bad things could happen to a thread in a pool

Do you ever wonder why threads in a thread pool can go bad and become unusable? Weblogic has long realized that. Hard to diagnose.

However, I don't believe "voodoo" exists in a threading system. A stuck thread must be stuck in a useless loop or waiting for cpu, IO or memory. It won't be stuck for "no reason". Pretend to be a spy looking at the program counter inside any thread -- why not moving? There are only a limited (not UNlimited) number of reasons. Here are various bad things that can happen to an innocent thread.

- in java, any uncaught exception can fall on a thread (like a rotten apple:). These are extremely common so a thread in a pool should handle these /gracefully/.

- a thread may be seen doing useful work in normal conditions, but in an edge case get into an endless loop but doing no useful work -- just wasting cpu
** Note endless loop is fine if the thread is doing useful work.

- divide by zero (perhaps terminating the thread)
- access wrong memory location and trigger a segfault. In C++ there are many pointer/memory-related errors.
- java thread can run out of memory too
- in java, a thread can be stopped due to the deprecated but supported stop() and suspend() methods. Such a thread might emerge in bad shape.
- starvation due to low thread priority.
- stack overflow, due to deep recursion (like a recursive equals() or hashCode()).

- thread could get stuck in a normal blocking operation like accept(), read(), write(), lock()/synchronized keyword, wait(), join(), long sleep. Many of these aren't even interruptible.
** IO (disk or network) wait is notorious culprit. In this state, everything including those "kill -9" signals are ignored (see [[Unix Power Tools]]). Entire process may appear stuck in mud.
** my java thread book also says a thread blocked on a socket may not respond to interrupt.
- deadlock

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