> On Mar 16, 2017, at 10:09 PM, Mark Lam <mark....@apple.com> wrote:
> 
> 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.

How much? What would be the new footprint with your suggestion below?

> 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.

Web Inspector’s interface wouldn’t look too good with such a large call stack 
anyway, so this doesn’t degrade the current experience IMO. Maybe even 50+50 
would be fine. I’ve never seen a call stack that large in JS code (but then 
again, I don’t use fancy frameworks).

> 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
> 
> 
> 
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