Hello JMeter users: Just wanted some feedback on a formula for max. active threads and some thoughts on errors when running tests with lots of threads.
Someone recently asked a question about JMeter running out of memory when creating new threads (the “cannot create native thread” error). I came up with this formula for maximum active threads in a thread group with a simple ramp *maximum active threads* = average thread lifetime * total threads/ramp period Does this look right? The way I understand the formula is it finds the total threads created by the time the first threads start terminating. Since threads now terminate at the same rate they are being added by the ramp, this is a steady-state figure - the “high water mark” of active threads. As for memory usage, each thread uses heap memory and stack memory. Total heap memory usage limits are defined in the JMeter run script by the ‘Xms’ (min.) and ‘Xmx’ (max.) flags. Stack memory usage is a fixed memory allocation per thread -- it can be changed by the ‘Xss’ parameter (but its best left alone in my experience) A 32-bit JVM running on a 32-bit OS can occupy 2 GB of RAM at most. As some RAM is taken up by the JVM runtime itself, a JMeter instance can really access only 1.5 -1.7 GB of RAM – the sum of both the heap and stack usage fit into this. So we do need to be careful about allocating too much heap. Lets assume *Xms* is set to *1.5 GB *and the test has a high water mark of 500 active threads. The JVM will probably run out of memory creating threads… even though there could be plenty of heap left unused, 500 threads require about 180 MB of stack (for a 32 bit JVM), and this will cause the test to probably generate a “cannot create native thread” error at some point. Regards Sonam
