Why is the JVM slow to start?

Question

What exactly makes the JVM (in particular, Sun\'s implementation) slow to get running compared to other runtimes like CPython? My impression was that it mainly has to do wit

Answer 1

In addition to things already mentioned (loading classes, esp. from compressed JARs); running in interpreted mode before HotSpot compiles commonly-used bytecode; and HotSpot compilation overhead, there is also quite a bit of one-time initialization done by JDK classes themselves. Many optimizations are done in favor of longer-running systems where startup speed is less of a concern.

And as to unix style pipelining: you certainly do NOT want to start and re-start JVM multiple times. That is not going to be efficient. Rather chaining of tools should happen within JVM. This can not be easily intermixed with non-Java Unix tools, except by starting such tools from within JVM.

Answer 2

If you are using Sun's HotSpot for x86_64 (64bit compiled), note that the current implementation only works in server mode, that is, it precompiles every class it loads with full optimization, whereas the 32bit version also supports client mode, which generally postpones optimization and optimizes the most CPU-intensive parts only, but has faster start-up times.

See for instance:

• http://en.wikipedia.org/wiki/64-bit#32_vs_64_bit
• http://java.sun.com/docs/hotspot/HotSpotFAQ.html#64bit_compilers

That being said, at least on my machine (Linux x86_64 with 64bit kernel), the 32bit HotSpot version supports both client and server mode (via the -client and -server flags), but defaults to server mode, while the 64bit version only supports server mode.