Why are Interpreted Languages Slow?

Question

I was reading about the pros and cons of interpreted languages, and one of the most common cons is the slowness, but why are programs in interpreted languages slow?

Answer 1

Native programs runs using instructions written for the processor they run on.

Interpreted languages are just that, "interpreted". Some other form of instruction is read, and interpreted, by a runtime, which in turn executes native machine instructions.

Think of it this way. If you can talk in your native language to someone, that would generally work faster than having an interpreter having to translate your language into some other language for the listener to understand.

Note that what I am describing above is for when a language is running in an interpreter. There are interpreters for many languages that there is also native linkers for that build native machine instructions. The speed reduction (however the size of that might be) only applies to the interpreted context.

So, it is slightly incorrect to say that the language is slow, rather it is the context in which it is running that is slow.

C# is not an interpreted language, even though it employs an intermediate language (IL), this is JITted to native instructions before being executed, so it has some of the same speed reduction, but not all of it, but I'd bet that if you built a fully fledged interpreter for C# or C++, it would run slower as well.

And just to be clear, when I say "slow", that is of course a relative term.

Answer 2

All answers seem to miss the real important point here. It's the detail how "interpreted" code is implemented.

Interpreted script languages are slower because their method, object and global variable space model is dynamic. In my opinion this is the real definition of of script language not the fact that it is interpreted. This requires many extra hash-table lookups on each access to a variable or method call. And its the main reason why they are all terrible at multithreading and using a GIL (Global Interpreter Lock). This lookups is where most of the time is spent. It is a painful random memory lookup, which really hurts when you get a L1/L2 cache-miss.

Google's Javascript Core8 is so fast and targeting almost C speed for a simple optimization: they take the object data model as fixed and create internal code to access it like the data structure of a native compiled program. When a new variable or method is added or removed then the whole compiled code is discarded and compiled again.

The technique is well explained in the Deutsch/Schiffman paper "Efficient Implementation of the Smalltalk-80 System".

The question why php, python and ruby aren't doing this is pretty simple to answer: the technique is extremely complicated to implement.

And only Google has the money to pay for JavaScript because a fast browser-based JavaScript interpreter is their fundamental need of their billion dollar business model.

Answer 3

Think of the interpeter as an emulator for a machine you don't happen to have

The short answer is that the compiled languages are executed by machine instructions whereas the interpreted ones are executed by a program (written in a compiled language) that reads either the source or a bytecode and then essentially emulates a hypothetical machine that would have run the program directly if the machine existed.

Think of the interpreted runtime as an emulator for a machine that you don't happen to actually have around at the moment.

This is obviously complicated by the JIT (Just In Time) compilers that Java, C#, and others have. In theory, they are just as good as "AOT" ("At One Time") compilers but in practice those languages run slower and are handicapped by needing to have the compiler around using up memory and time at the program's runtime. But if you say any of that here on SO be prepared to attract rabid JIT defenders who insist that there is no theoretical difference between JIT and AOT. If you ask them if Java and C# are as fast as C and C++, then they start making excuses and kind of calm down a little. :-)

So, C++ totally rules in games where the maximum amount of available computing can always be put to use.

On the desktop and web, information-oriented tasks are often done by languages with more abstraction or at least less compilation, because the computers are very fast and the problems are not computationally intensive, so we can spend some time on goals like time-to-market, programmer productivity, reliable memory-safe environments, dynamic modularity, and other powerful tools.