[webkit-dev] Proposal to limit the size of the captured exception stack

[Mark Lam]

Hi folks,

Currently, if we have an exception stack that is incredibly deep (especially for a StackOverflowError), JSC may end up thrashing memory just to capture the large stack trace in memory.    This is bad for many reasons:

1. the captured stack will take a lot of memory.
2. capturing the stack may take a long time (due to memory thrashing) and makes for a bad user experience.
3. if memory availability is low, capturing such a large stack may result in an OutOfMemoryError being thrown in its place.
    The OutOfMemoryError thrown there will also have the same problem with capturing such a large stack.
4. most of the time, no one will look at the captured Error.stack anyway.

Since there isn’t a standard on what we really need to capture for Error.stack, I propose that we limit how much stack we capture to a practical size.  How about an Error.stack that consists of (1) the top N frames, (2) an ellipses, and (3) the bottom M frames?  If the number of frames on the stack at the time of capture  is less or equal to than N + M frames, then Error.stack will just show the whole stack with no ellipses.  For example, if N is 4 and M is 2, the captured stack will look something like this:

      foo10001
      foo10000
      foo9999
      foo9998
      …
      foo1
      foo0

If we pick a sufficient large number for N and M (I suggest 100 each), I think this should provide sufficient context for debugging uses of Error.stack, while keeping an upper bound on how much memory and time we throw at capturing the exception stack.

My plan for implementing this is:
1. change Exception::finishCreation() to only capture the N and M frames, plus possibly 1 ellipses placeholder in the between them.
2. change all clients of Exception::stack() to be able to recognize and render the ellipses.

Does anyone object to doing this or have a compelling reason why this should not be done?

Thanks.

Mark