[webkit-dev] Proposal to limit the size of the captured exception stack
Mark Lam
mark.lam at apple.com
Fri Mar 17 13:04:29 PDT 2017
@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 <mjs at apple.com> wrote:
>
>>
>> On Mar 17, 2017, at 11:09 AM, Mark Lam <mark.lam at apple.com <mailto:mark.lam at 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 <ggaren at apple.com <mailto: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 <mailto: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
>>>>
>>>>
>>>>
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>>>
>>
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