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Hey guys,

As I said in my previous post, aside from my main work on a smalltalk research virtual machine, I decided 3 months ago to allocate 1/5th of my time to work on the Cog VM. After a discussion with its core designer and developer, Eliot Miranda, it was decided that I will spend time on Cog’s adaptive recompiler: Sista.

In this post and on all future posts about Sista, I will talk about what I am implementing. Therefore, it may be that I will not talk about the best solution for a specific problem, but I will talk about a solution that works and that is good enough. It may also occur that what I am doing is going in the wrong direction and that part of the project may, one day or the other, fail. However, I’m very confident in the success of the Sista, this is why I start this blog post chronicles.

Plan

  • What is an adaptive recompiler ?
  • What is the Sista approach ?
  • What are the different steps, is there release date ?

What is an adaptive recompiler ?

An adaptive recompiler is a tool that translate on-the-fly at runtime a code snippet into another code snippet that is faster to execute, based on analysis of the environment and the previous executions of the code snippet.

Even if a software application is big, usually at most 10% of its code is frequently used. Therefore, the optimizations, wasting memory in exchange for speed, should focus on this 10%, and not on 100% of the system. Efficient virtual machines feature a hot spot detector, a tool that permits to detect portions of code that are frequently used. After a few iterations, the hot spots are converted from virtual code (such as byte code) to native code, to remove the interpretation overhead and to add inline caches. Then, based on the first executions, efficient virtual machines have an adaptive recompiler, that recompiles these portions of code into a faster code. The adaptive recompiler optimization process is triggered by the JIT on very hot spots. For example, java hot spot or javascript V8 have an adaptive recompiler as part of their JIT infrastructure.

Aside from optimizing methods, an adaptive recompiler should have a deoptimizing infrastructure, to allow the user to debug the code as if the adaptive recompiler was not present and to deoptimize then reoptimize a method if its common execution case has changed (the code was firstly optimized based on its first executions, but if the use case of the first execution become obsolete, you need to recompile again the method in a different way).

To understand well how things work, let’s sum up how a VM usually optimize portions of code:

  • step 1 (portions of code rarely used): the code is interpreted
  • step 2 (portions of code sometimes used): the code has been compiled on-the-fly to native code and the native code is run
  • step 3 (portions of code used often): the code has been recompiled into a better native code with optimizations *fast* to execute based on previous executions common cases
  • step 4 (portions of code used almost all the time): the code has been recompiled into a better native code with optimizations *slow* to execute, based on previous executions common cases

The difference between the step 3 and step 4 is that in step 3 you will perform optimizations that are quickly perform, therefore the code is quickly recompiled into something better, whereas in step 4 the generated code will be faster but the recompilation may take a while, so you need to be sure that this hot spot is really really hot.

Lastly, the activation of these optimized methods is quite complex. Let’s say you have the method:

Interpreter>>interpret
[interpreter interpretNextInstruction] repeat

Clearly in this case the block will be executed a lot of time, so we want to optimize it. Now, if we only compile an optimized version of this method and wait for the next activation of the method for the optimized method to be run, the optimization would have been useless, because it is very unlikely that the interpreter will stop interpreting code, therefore it will always use the same unoptimized old method of the first activation. It may also happen that you run a method millions of times to do a specific operation, and then never use it again. Therefore, optimizing a method and then wait for it next execution to run the optimized code is not good enough. The solution for this problem is to edit the stack after recompiling a method into an optimized one, in order to remove unoptimized stack frames/contexts and replace them by an optimized context.

What is the Sista approach ?

With the previous ideas I explained, one can understand that the Sista may greatly improve the speed of Pharo. Of course I’m not going to gain 100x boost, because we are only 2 guys spending part of our time on this project (me with the help of Eliot). But I believe we can get some performance boost.

In the case of Pharo and Cog, the Sista requires 2 tools:

  • a hot spot detector
  • an optimizer/deoptimizer

Currently, in Cog’s JIT, methods are compiled to native code on-the-fly when they are called if they’re already on the global lookup cache. This process is good enough to detect portions of code needing to be jitted (it detects code executed 2 or 3 times in a certain amount of time). However, this is not good enough for the adaptive recompiler, which needs to know how many times the portion of code was executed, detecting a hot spot for example when a method is executed several thousands times in a certain amount of time. On the other hand, the hot spot detector has to detect methods that are frequently used without reducing too much the overall performance of the system. Eliot Miranda had already a good prototype for this. Basically, the Sista Cogit adds counters in some places in the native code, and when a counter reach a certain limit, it triggers the adaptive recompiler. These counters add a few overhead in the code execution (seemingly 15% overhead), but the adaptive recompiler will allow us to earn much more performance than the few that is lost.

The optimizer/deoptimizer has to be able to recompile methods, optimizing them with type feed-back result from the native code inline caches. It was decided, to make it as simple as possible, to implement this part in the Smalltalk image. Therefore, a virtual machine hook will trigger the in-image optimizer/deoptimizer from the JIT.

In the beginning, the Sista will have only 1 optimization step: the methods will be recompiled on the fly after 30000 executions.

sista overview

On hot spot detection, the in-image optimizer will analyse the stack, find a good method to optimize, optimize it, edit the stack to put the optimized method instead of the slower method, and restart the execution. The method is optimized based on the inline caches data which provides type-feedback information. The optimized method has access to an extended byte code set with extras byte codes tipping the JIT on how to produce better machine code.

What are the different steps, is there release date ?

Currently a prototype of the hotspot detector is working. I guess Eliot will finish it at some point if I finish something in the image :).

On my side, I am working on the in-image optimizer infrastructure, which I will introduce in a future post (Sista chronicles III). Then I’ll work on a few optimizations (step 1 described later in this post). The next step will be to fix the on-the-fly stack frames replacement (when recompiling a method, you need to edit the current execution state so it uses the optimized method and not the old one) and work on the deoptimization process. Last step is to add more optimizations and to add some kind of dependency management in the image (as you know, in Smalltalk, we always compile method at runtime, therefore we need to unoptimize methods that had inlined the method freshly recompiled).

Here are the optimizations I plan to implement and their effect for Pharo users:

  • step 1: basic inlining of quick methods, methods, block. Here users will gain performance from block inlining.
  • step 2: basic dead code elimination and copy propagation. Here the user will gain additional performance.
  • step 3: new bytecode set, inlining of primitives, addition of unchecked primitive. Here the users will gain both performance and flexibility in the system (for example, this fix will remove the 255 max instance variable limitation and increase the maximum number of literals of a method).

Then, one could look at:

  • very hot spot optimizations, such as loop unrolling, instruction scheduling mixed with tail called recursion, …
  • polymorphic inline cache optimizations, such as message splitting to improve the quality of inlining and method customization
  • improvement of all the algorithms in general, I am implementing all of them in a working but not perfect way, so for example people could improve the SSA / deSSA algorithm.
  • Floating-pointer fast arithmetic

Hope you enjoyed this first post 🙂

Note: Not so smart questions

Virtual machine is a very specific domain, requiring very deep knowledge to understand it fully. Some virtual machine optimizations are *not* intuitive at all. For example, a few time ago, I thought that increasing the size of the native code cache of Cog JIT would speed up the system, because native code is faster to execute than interpreted code, so having more would speed more the system. But it is not the case at all, because the cpu instruction cache logic has problem with a big amount of native code. Therefore having only hot spots in native code and the rest of the code interpreted is better in the current Cog VM (growing the cache from 1Mb to 2Mb decrease the performance by around 40%). Due to its specificity, people that does not know a lot about virtual machine ask always the same questions and *cannot easily* understand the answers. To have answers about these kind of questions, please read the virtual machines literature. Therefore I will not validate nor answer questions in comments (in all the Sista chronicles) that does not go into a productive direction, but instead I forward you to the virtual machines literature. For example, I will not allow questions such as: “I don’t believe in adaptive recompilers. Why don’t you do these optimizations in a static compiler with type inference algorithm ?”.

But don’t worry, you can find answers by yourself about that in the virtual machine literature ;-). Look for example at the Self VM papers to understand why adaptive recompilation is so useful.

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