> On Mar 28, 2017, at 4:23 PM, Geoffrey Garen <gga...@apple.com> wrote:
> 
> Does the separate exceptionStackTraceLimit mean that if a developer gets a 
> truncated stack trace in the Web Inspector, there’s no way for the developer 
> to remedy that? Is that what other browsers’ developer tools do?

FireFox and Chrome show console entires with exception stack traces with 128 
and 200 frames (respectively).

> 
> Geoff
> 
>> On Mar 28, 2017, at 4:09 PM, Mark Lam <mark....@apple.com 
>> <mailto:mark....@apple.com>> wrote:
>> 
>> To follow up, I’ve implemented the change in r214289: <http://trac.webkit 
>> <http://trac.webkit/>.org/r214289>.  Error.stackTraceLimit is now 100.  I 
>> also implemented a separate exceptionStackTraceLimit for stack traces 
>> captured at the time of throwing a value (not to be confused with 
>> Error.stack which is captured at the time of instantiation of the Error 
>> object).  exceptionStackTraceLimit is also limited to 100 by default.
>> 
>> Mark
>> 
>> 
>>> On Mar 17, 2017, at 1:04 PM, Mark Lam <mark....@apple.com 
>>> <mailto:mark....@apple.com>> wrote:
>>> 
>>> @Geoff, my testing shows that we can do 200 frames and still perform well 
>>> (~1 second to console.log Error.stack).  Base on what we at present, I 
>>> think 100 is a good round number to use as our default stackTraceLimit.
>>> 
>>>> On Mar 17, 2017, at 11:40 AM, Maciej Stachowiak <m...@apple.com 
>>>> <mailto:m...@apple.com>> wrote:
>>>> 
>>>>> 
>>>>> On Mar 17, 2017, at 11:09 AM, Mark Lam <mark....@apple.com 
>>>>> <mailto:mark....@apple.com>> wrote:
>>>>> 
>>>>> 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.
>>>> 
>>>> Out of curiosity: Why does Firefox capture 129 frames instead of 31722 in 
>>>> this case? Do they have a hardcoded limit?
>>> 
>>> Actually, my previous frame counts are a bit off.  I was using 
>>> e.stack.split(/\r\n|\r|\n/).length as the frame count.  Below, I just copy 
>>> the console.log dump into an editor and take the line count from there as 
>>> the frame count instead.  The result of that string.split appears to be a 
>>> bit off from the actual frames printed by console.log. 
>>> 
>>> I also modified my recursing test function to console.log the re-entry 
>>> count on entry and this is what I saw:
>>> 
>>> 1. Chrome
>>>     test reported reentry count = 10150
>>>     ....split(…).length = 11 (because Chromes starts e.stack with a line 
>>> "RangeError: Maximum call stack size exceeded”)
>>>     e.stack lines according to editor = 10 frames
>>> 
>>> 2. Firefox
>>>     test reported reentry count = 222044
>>>     ....split(…).length = 129 (probably because there’s an extra newline in 
>>> there somewhere)
>>>     e.stack lines according to editor = 128 frames
>>> 
>>> 3. Safari
>>>     test reported reentry count = 31701
>>>     ....split(…).length = 31722 (I don’t know why there’s a 21 frame 
>>> discrepancy here.  I’ll debug this later)
>>>     e.stack lines according to editor = ??? frames (WebInspector hangs 
>>> every time I try to scroll in it, let alone let me highlight and copy the 
>>> stack trace.  So I gave up)
>>> 
>>> Assuming the test function frame is not significantly different in size for 
>>> all browsers, it looks like:
>>> 1. Chrome uses a much smaller stack (about 1/3 of our stack).
>>> 2. Firefox uses a much larger stack (possibly the full machine stack), but 
>>> caps its Error.stack to just 128 frames (possibly a hardcoded limit).
>>> 
>>> Mark
>>> 
>>> 
>>>> 
>>>>  - Maciej
>>>> 
>>>>> 
>>>>> Does anyone object to us adopting Error.stackTraceLimit and setting the 
>>>>> default to 10 to match Chrome?
>>>>> 
>>>>> Mark
>>>>> 
>>>>> 
>>>>> 
>>>>>> On Mar 16, 2017, at 11:29 PM, Geoffrey Garen <gga...@apple.com 
>>>>>> <mailto:gga...@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....@apple.com 
>>>>>>> <mailto: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.
>>>>>>> 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
>>>>>>> 
>>>>>>> 
>>>>>>> 
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>>>>>> 
>>>>> 
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