Bug 158678 – The sampling profiler should further protect itself against certain forms of sampling bias that arise due to the sampling interval being in sync with some other system process

• Component: JavaScriptCore
• Status: RESOLVED FIXED

[Saam Barati]

This problem was first brought to my attention after reading this paper.
See section 6.2:
http://plv.colorado.edu/papers/mytkowicz-pldi10.pdf

It seems somewhat rare that this will happen in practice, and it's
unlikely to happen many times in a row after restarting the profiler,
but it's good practice to defend against this form of bias.

[Saam Barati]

Created attachment 281176 [details]
patch

[WebKit Commit Bot]

Attachment 281176 [details] did not pass style-queue:


ERROR: Source/JavaScriptCore/runtime/SamplingProfiler.cpp:52:  Alphabetical sorting problem.  [build/include_order] [4]
Total errors found: 1 in 2 files


If any of these errors are false positives, please file a bug against check-webkit-style.

[Benjamin Poulain]

Comment on attachment 281176 [details]
patch

View in context: https://bugs.webkit.org/attachment.cgi?id=281176&action=review

> Source/JavaScriptCore/runtime/SamplingProfiler.cpp:237
> +        std::chrono::microseconds randomFluctuation = std::chrono::microseconds(static_cast<uint64_t>(randomNumber() * (static_cast<double>(m_timingInterval.count()) * 0.20l)));

Is this right?

I understand a distribution around "t" but randomNumber() is always positive, m_timingInterval.count() is always positive.
Shouldn't the distribution be spread on both sides of t?

This line would also benefit from mentioning the paper in a comment.

[Saam Barati]

Comment on attachment 281176 [details]
patch

View in context: https://bugs.webkit.org/attachment.cgi?id=281176&action=review

>> Source/JavaScriptCore/runtime/SamplingProfiler.cpp:237
>> +        std::chrono::microseconds randomFluctuation = std::chrono::microseconds(static_cast<uint64_t>(randomNumber() * (static_cast<double>(m_timingInterval.count()) * 0.20l)));
> 
> Is this right?
> 
> I understand a distribution around "t" but randomNumber() is always positive, m_timingInterval.count() is always positive.
> Shouldn't the distribution be spread on both sides of t?
> 
> This line would also benefit from mentioning the paper in a comment.

Yeah, obviously. I wasn't thinking. This is definitely wrong.
I shall fix and update with a link. Thanks for catching this

[Saam Barati]

Created attachment 281221 [details]
patch

[WebKit Commit Bot]

Attachment 281221 [details] did not pass style-queue:


ERROR: Source/JavaScriptCore/runtime/SamplingProfiler.cpp:52:  Alphabetical sorting problem.  [build/include_order] [4]
Total errors found: 1 in 2 files


If any of these errors are false positives, please file a bug against check-webkit-style.

[Benjamin Poulain]

Comment on attachment 281221 [details]
patch

View in context: https://bugs.webkit.org/attachment.cgi?id=281221&action=review

> Source/JavaScriptCore/runtime/SamplingProfiler.cpp:241
> +        double randomSignedNumber = (randomNumber() * 2.0l) - 1.0l; // A random number between [-1, 1).

What's the .01?
Why not "(randomNumber() * 2.) - 1"?

> Source/JavaScriptCore/runtime/SamplingProfiler.cpp:242
> +        std::chrono::microseconds randomFluctuation = std::chrono::microseconds(static_cast<uint64_t>(randomSignedNumber * static_cast<double>(m_timingInterval.count()) * 0.20l));

What's that "* 0.20l" factor here?

[Saam Barati]

Comment on attachment 281221 [details]
patch

View in context: https://bugs.webkit.org/attachment.cgi?id=281221&action=review

>> Source/JavaScriptCore/runtime/SamplingProfiler.cpp:241
>> +        double randomSignedNumber = (randomNumber() * 2.0l) - 1.0l; // A random number between [-1, 1).
> 
> What's the .01?
> Why not "(randomNumber() * 2.) - 1"?

it's an 'L' not a 1.
For a double literal. I'll just switch to 2.0 and 1.0

>> Source/JavaScriptCore/runtime/SamplingProfiler.cpp:242
>> +        std::chrono::microseconds randomFluctuation = std::chrono::microseconds(static_cast<uint64_t>(randomSignedNumber * static_cast<double>(m_timingInterval.count()) * 0.20l));
> 
> What's that "* 0.20l" factor here?

We add +-(0.2t) where t is our sampling interval.
We're just choosing some range between [-t, t]

[Saam Barati]

Created attachment 281223 [details]
patch

[Saam Barati]

(In reply to comment #8)
> Comment on attachment 281221 [details]
> patch
> 
> View in context:
> https://bugs.webkit.org/attachment.cgi?id=281221&action=review
> 
> >> Source/JavaScriptCore/runtime/SamplingProfiler.cpp:241
> >> +        double randomSignedNumber = (randomNumber() * 2.0l) - 1.0l; // A random number between [-1, 1).
> > 
> > What's the .01?
> > Why not "(randomNumber() * 2.) - 1"?
> 
> it's an 'L' not a 1.
> For a double literal. I'll just switch to 2.0 and 1.0
> 
> >> Source/JavaScriptCore/runtime/SamplingProfiler.cpp:242
> >> +        std::chrono::microseconds randomFluctuation = std::chrono::microseconds(static_cast<uint64_t>(randomSignedNumber * static_cast<double>(m_timingInterval.count()) * 0.20l));
> > 
> > What's that "* 0.20l" factor here?
> 
> We add +-(0.2t) where t is our sampling interval.
> We're just choosing some range between [-t, t]

To elaborate on this, we don't do the full
[-t, t] because we want to defend against longer sample
times for short running programs. Even though short
running programs are hard to run under a sampling profiler,
we don't want to hurt such uses. We choose 0.2 because
a random 20% of our default (1ms) range is enough to add 
randomness to our sample times so we don't end up
in sync with some other system process.

[WebKit Commit Bot]

Comment on attachment 281223 [details]
patch

Clearing flags on attachment: 281223

Committed r202021: <http://trac.webkit.org/changeset/202021>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.