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

Mark Lam mark.lam at apple.com
Fri Mar 17 11:09:34 PDT 2017

Thanks for the reminder to back observations up with data.  I was previously running some tests that throws StackOverflowErrors a lot (which tainted my perspective), and I made a hasty conclusion which isn’t good.  Anyway, here’s the data using an instrumented VM to take some measurements and a simple test program that recurses forever to throw a StackOverflowError (run on a MacPro):

1. For a release build of jsc shell:
    Time to capture exception stack = 0.002807 sec
    Number of stack frames captured = 31722
    sizeof StackFrame = 24
    total memory consumed = ~761328 bytes.

2. For a debug build of jsc shell:
    Time to capture exception stack = 0.052107 sec
    Number of stack frames captured = 31688
    sizeof StackFrame = 24
    total memory consumed = ~760512 bytes.

So, regarding performance, I was wrong.  The amount of time taken to capture the entire JS stack each time is insignificant.
Regarding memory usage, ~760K is not so good, but maybe it’s acceptable.

Comparing browsers with their respective inspectors open:

1. Chrome
    number of frames captured: 10
    length of e.stack string: 824 chars
    time to console.log e.stack: 0.27 seconds

2. Firefox
    number of frames captured: 129
    length of e.stack string: 8831 chars
    time to console.log e.stack: 0.93 seconds

3. Safari
    number of frames captured: 31722
    length of e.stack string: 218821 chars
    time to console.log e.stack: 50.8 seconds

4. Safari (with error.stack shrunk to 201 frames at time of capture to simulate my proposal)
    number of frames captured: 201
    length of e.stack string: 13868 chars
    time to console.log e.stack: 1 second

With my proposal, the experience of printing Error.stack drops from 50.8 seconds to about 1 second.  The memory used for capturing the stack also drops from ~760K to 5K.

I wasn’t aware of the Error.stackTraceLimit, but that does sound like a better solution than my proposal since it gives developers the ability to capture more stack frames if they need it.  Chrome’s default Error.stackTraceLimit appears to be 10.  MS appears to support it as well and defaults to 10 (https://docs.microsoft.com/en-us/scripting/javascript/reference/stacktracelimit-property-error-javascript <https://docs.microsoft.com/en-us/scripting/javascript/reference/stacktracelimit-property-error-javascript>).  Firefox does now.

Does anyone object to us adopting Error.stackTraceLimit and setting the default to 10 to match Chrome?


> On Mar 16, 2017, at 11:29 PM, Geoffrey Garen <ggaren at apple.com> wrote:
> Can you be more specific about the motivation here?
> Do we have any motivating examples that will tell us wether time+memory were unacceptable before this change, or are acceptable after this change?
> In our motivating examples, does Safari use more time+memory than other browsers? If so, how large of a stack do other browsers capture?
> We already limit the size of the JavaScript stack to avoid performance problems like the ones you mention in many other contexts. Why is that limit not sufficient?
> Did you consider implementing Chrome’s Error.stackTraceLimit behavior?
> Geoff
>> On Mar 16, 2017, at 10:09 PM, Mark Lam <mark.lam at 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.
>> 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
>> _______________________________________________
>> webkit-dev mailing list
>> webkit-dev at lists.webkit.org
>> https://lists.webkit.org/mailman/listinfo/webkit-dev

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