JVM + Linux + Intel's Hyperthreading =

Question

I noticed that JVM threads run as processes under Linux for some reasons (correct me if I'm wrong). Furthermore, it is a fact that Intel's Hyperthreading provides additional parallelization only for two threads belonging to same process.

Does that mean that a single multitheraded JVM program would not profit from Hyperthreading under Linux, because its threads are no threads from the CPUs "point of view"?

Answer 1

Processes and threads are not treated differently by the scheduler in Linux. There are a range of resources that can be shared by processes, as defined by the clone system call. Threads and processes as they're typically used are just names for commonly used recipes.

If you're observing threads as processes in the JVM, this is just a mixing of nomenclature. By the usual definition if processes are sharing a virtual address space, then they are "threads" within a process.

All hardware scheduling will benefit from hyper-threading, regardless of the terminology used. Also to be completely fair, hyper threading does not allow you to run more threads in parallel: it makes context switches faster, giving processes more run time.

Answer 2

"JVM threads run as processes under Linux "-- No they run as LWP(Light Weight Process).

Java threads are internally implemented as native threads i.e. LWP (in linux) and you can see them using ps -eLf. Though mapping between a native thread and a java thread is difficult. The only thread that can be mapped easily is the main-thread as it will have the id same as the process id.

JVM will definitely profit from HT.
From an article on HT in java:

SMT holds the promise of significantly increasing Java's server-side performance by more completely utilizing existing processor cycles in multithreaded applications.