Why ulimit can't limit resident memory successfully and how?
According to the man page for setrlimit:
RLIMIT_RSS
Specifies the limit (in pages) of the process's resident set (the number of virtual pages resident in RAM). This limit only has effect in Linux 2.4.x, x < 30, and there only affects calls to madvise(2) specifying MADV_WILLNEED
You probably want to set the virtual memory size instead, via ulimit -v