Hi all,
We started (with many interruptions over the last months) working a bit with Stephane on understanding what is the (positive and negative) impact of stack-overflow support in Pharo.
The key idea is that if a process consumes too much stack (potentially because of an infinite recursion) then the process should stop with an exception.
## Why we want better stack consumption control
This idea comes up to solve issues that are pretty common and hit especially newbies.
For example, imagine you accidentally write an accessor such as
```
A >> foo
^ self foo
```
Students do this all the time, and Iâve also seen it in experienced people who go too fast :).
More importantly, such recursions could happen also with not-so-obvious indirect recursions (a sends b, b sends c, c sends a), and these could hit anybody.
This is aggravated because the current execution model allows us to have infinite stacks âmeaning: limited by available memory only.
This is indeed a nice feature for many use cases but it has its own drawbacks when one of these kind of recursions are hit:
- code just loops forever taking space in the stack
- when there is no more stack space, context objects are created and moved to the heap
- but those contexts are strongly held, so they are never GCed and take up extra space
- even worse! they are there adding more work to the GC every time and making the GC run more often looking for space that is not there
## Why Ctrl-dot does not always work
Of course, super users know there is this âCtrl dotâ hidden feature that should help you recover from this.
First, let's take out of the equation that this is only known by super users.
Now, in this situation, when Ctrl-dot is hit it will trigger a handler that suspends the problematic process and opens a debugger on it.
But it could happen that,
- the stack is so big that the debugger is very sluggish (best-case scenario)
- the VM is just flooded doing GCs so maybe the Ctrl dot event does not even arrive at Pharo or the trigger
- if the recursion is hit when printing an object (which is more common than you could imagine), opening the debugger could trigger a new recursion and never give back the control to the user
## What are we working on
The main idea here is: Can we have a simple and efficient way to prevent such kinds of situations?
After many discussions around detecting recursion, we kinda arrived at the simple solution of just detecting a stack overflow.
The solution is easy to understand (because itâs like other languages work) and easy to implement because there is already support for that.
But this leaves open two questions:
- what happens when people want to use the âinfinite stackâ feature?
- when should a process stack overflow? What is a sensitive default value?
Our draft implementation here https://github.com/pharo-project/pharo-vm/pull/710 <https://github.com/pharo-project/pharo-vm/pull/710> does the following to cope with this:
- we can now parametrize the size of the stack (of each stack page to be more accurate) when the VM starts up
- the stack overflow check can be disabled per process
We also are running experiments to see what could be a sensitive stack size for our normal usages. Here, for example, we ran almost all test cases in Pharo separately (one suite per line below), and we observed how many tests broke (x-axis) with different stack sizes (y-axis).
Here we see that most test suites require at least 20-24k to run properly, some go up to 36k of stack before converging (i.e., the number of broken tests does not change).
Youâll notice in the graph that There are some scenarios that break all the time. This is because exception handling itself is recursive and may produce more stack overflows depending on the size of the stack between the exception and the exception handler.
So some more work is still required, mostly changing Pharo libraries to properly support this. For example:
- should tests run in a fresh process with a fresh stack?
- should the exception mechanism use less recursion?
- resumable exceptions add stack pressure because they do not âunstackâ until the exception is finally handled, meaning that the stack used by exception handling just adds up to the stack of the original code, can we do better here?
Probably there are more interesting questions here, thatâs the âwhy" behind this email.
Iâm interested in opinions and scenarios you may come up with that should be taken into account.
Cheers,
Guille
