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

2017-03-29 Thread Matt Baker
> On Mar 28, 2017, at 9:55 PM, Mark Lam  wrote:
> 
> Matt,
> I tested this on Firefox and Chrome and saw that Chrome captures up to 200 
> frames.  I don’t see Firefox capturing frames at all when not throwing an 
> Error.  Were you looking at Error.stack for Firefox when you came up with the 
> 128 frames number?  Maybe there’s a Firebug option I’m not familiar with?

The 128 frames is what I observed in Firefox’s console after loading a page 
which runs a script recursively in order to trigger a stack overflow. Enabling 
“Pause on Exceptions” in Firefox didn’t cause the debugger to pause when 
throwing the error (same as Chrome) so I’m not sure how to interact with the 
Error object.

> Geoff,
> It just occurred to me that the developer does have one recourse: he/she can 
> use break on exception thrown / on uncaught exception, and inspect the full 
> stack via the debugger.
> 
> Mark
> 
> 
>> On Mar 28, 2017, at 9:08 PM, Mark Lam > > wrote:
>> 
>> Currently, there’s no way for the developer to change this.  We can 
>> certainly make it an option that the Inspector can change if desired.
>> 
>> Mark
>> 
>> 
>>> On Mar 28, 2017, at 7:35 PM, Matt Baker >> > wrote:
>>> 
 On Mar 28, 2017, at 4:23 PM, Geoffrey Garen > 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  > wrote:
> 
> To follow up, I’ve implemented the change in r214289:  .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 > > 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 >> > wrote:
>>> 
 
 On Mar 17, 2017, at 11:09 AM, Mark Lam > 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)

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

2017-03-28 Thread Mark Lam
Matt,
I tested this on Firefox and Chrome and saw that Chrome captures up to 200 
frames.  I don’t see Firefox capturing frames at all when not throwing an 
Error.  Were you looking at Error.stack for Firefox when you came up with the 
128 frames number?  Maybe there’s a Firebug option I’m not familiar with?

Geoff,
It just occurred to me that the developer does have one recourse: he/she can 
use break on exception thrown / on uncaught exception, and inspect the full 
stack via the debugger.

Mark


> On Mar 28, 2017, at 9:08 PM, Mark Lam  wrote:
> 
> Currently, there’s no way for the developer to change this.  We can certainly 
> make it an option that the Inspector can change if desired.
> 
> Mark
> 
> 
>> On Mar 28, 2017, at 7:35 PM, Matt Baker > > wrote:
>> 
>>> On Mar 28, 2017, at 4:23 PM, Geoffrey Garen >> > 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 > wrote:
 
 To follow up, I’ve implemented the change in r214289: .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  > 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 > > wrote:
>> 
>>> 
>>> On Mar 17, 2017, at 11:09 AM, Mark Lam >> > 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 

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

2017-03-28 Thread Mark Lam
Currently, there’s no way for the developer to change this.  We can certainly 
make it an option that the Inspector can change if desired.

Mark


> On Mar 28, 2017, at 7:35 PM, Matt Baker  wrote:
> 
>> On Mar 28, 2017, at 4:23 PM, Geoffrey Garen > > 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 >> > wrote:
>>> 
>>> To follow up, I’ve implemented the change in r214289: >> .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 > 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  > wrote:
> 
>> 
>> On Mar 17, 2017, at 11:09 AM, Mark Lam > > 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
>>  
>> ).
>>   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 

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

2017-03-28 Thread Matt Baker
> On Mar 28, 2017, at 4:23 PM, Geoffrey Garen  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 > > wrote:
>> 
>> To follow up, I’ve implemented the change in r214289: > .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 >> > 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 > wrote:
 
> 
> On Mar 17, 2017, at 11:09 AM, Mark Lam  > 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
>  
> ).
>   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 

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

2017-03-28 Thread Geoffrey Garen
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?

Geoff

> On Mar 28, 2017, at 4:09 PM, Mark Lam  wrote:
> 
> To follow up, I’ve implemented the change in r214289:  .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 > > 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 >> > wrote:
>>> 
 
 On Mar 17, 2017, at 11:09 AM, Mark Lam > 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
  
 ).
   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 

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

2017-03-28 Thread Mark Lam
To follow up, I’ve implemented the change in r214289: .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  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 > > wrote:
>> 
>>> 
>>> On Mar 17, 2017, at 11:09 AM, Mark Lam >> > 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
>>>  
>>> ).
>>>   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)
> 
> 

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

2017-03-17 Thread Mark Lam
@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  wrote:
> 
>> 
>> On Mar 17, 2017, at 11:09 AM, Mark Lam > > 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
>>  
>> ).
>>   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 >> > wrote:
>>> 
>>> Can you be more specific about the motivation here?
>>> 
>>> Do we have any motivating 

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

2017-03-17 Thread Maciej Stachowiak

> On Mar 17, 2017, at 11:09 AM, Mark Lam  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
>  
> ).
>   Firefox does now.

Out of curiosity: Why does Firefox capture 129 frames instead of 31722 in this 
case? Do they have a hardcoded limit?

 - 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 > > 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 >> > 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:

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

2017-03-17 Thread Geoffrey Garen
Thanks for the detailed write-up.

The main thing that sticks out to me in this data is that Safari defaults to 
capturing a stack that is, in the worst case, roughly 3000X larger than the 
stack in IE and Chrome. That’s a big difference. I think this could be a real 
website compatibility problem since an author who tested in IE or Chrome might 
not notice a repeatedly thrown exception, which could cause time or memory or 
even bandwidth problems (if the author phoned home with exception data) in 
Safari.

Since Chrome and IE have already adopted it, I like Error.stackTraceLimit, and 
I think we should propose standardizing it. (We can standardize 
Error.stackTraceLimit even before we fully standardize the text content of 
Error.stack.)

I think 10 is possibly an unnecessarily low default. Most stack traces are 
bigger than 10. 30 would still be two orders of magnitude better than the 
status quo. Even 50 or 100 might be OK, as long as your testing shows it’s not 
too expensive.

Elipsizing was a cool idea, but given the behavior of other browsers, I think 
it’s better to truncate Error.stack and consider elipsizing in Web Inspector, 
where presentation matters more and has less of an effect on web compatibility.

Geoff

> On Mar 17, 2017, at 11:09 AM, Mark Lam  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
>  
> ).
>   Firefox does now.
> 
> 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 > > 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 >> > 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 

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

2017-03-17 Thread Mark Lam
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
 
).
  Firefox does now.

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  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  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
>> foo1
>> foo
>> 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 

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

2017-03-17 Thread Geoffrey Garen
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  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
>  foo1
>  foo
>  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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Re: [webkit-dev] Proposal to limit the size of the captured exception stack

2017-03-16 Thread Brian Burg

> On Mar 16, 2017, at 10:09 PM, Mark Lam  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
>  foo1
>  foo
>  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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[webkit-dev] Proposal to limit the size of the captured exception stack

2017-03-16 Thread 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
  foo1
  foo
  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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