V8 Installation and d8 shell usage

v8.md

Installing V8 on a Mac

Prerequisites

• Install Xcode (Avaliable on the Mac App Store)
• Install Xcode Command Line Tools (Preferences > Downloads)
• Install depot_tools
  • git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git
  • sudo nano ~/.bash_profile
  • Add export PATH=/path/to/depot_tools:"$PATH" (it's important that depot_tools comes first here)
  • source ~/.bash_profile
  • From the directory you want to install V8 into, run gclient

Build V8

• fetch v8
• cd v8
• gclient sync
• tools/dev/v8gen.py x64.optdebug
• ninja -C out.gn/x64.optdebug (prepare for lots of fan noise)

I'd also recommend adding these to your .bash_profile:

• sudo nano ~/.bash_profile
• Add alias d8=/path/to/v8/repo/out.gn/x64.optdebug/d8
• Add alias tick-processor=/path/to/v8/repo/tools/mac-tick-processor
• Add export D8_PATH="/path/to/v8/repo/out.gn/x64.optdebug"
• source ~/.bash_profile

d8 shell examples

Print optimization stats

Create test.js with the following code:

function test( obj ) {
  return obj.prop + obj.prop;
}

var a = { prop: 'a' }, i = 0;

while ( i++ < 10000 ) {
  test( a );
}

Run d8 --trace-opt-verbose test.js

You should see that the test function was optimized by V8, along with an explanation of why. "ICs" stands for inline caches -- and are one of the ways that V8 performs optimizations. Generally speaking, the more "ICs with typeinfo" the better.

Now modify test.js to include the following code:

function test( obj ) {
  return obj.prop + obj.prop;
}

var a = { prop: 'a' }, b = { prop: [] }, i = 0;

while ( i++ < 10000 ) {
  test( Math.random() > 0.5 ? a : b );
}

Run d8 --trace-opt-verbose test.js

So, you'll see that this time, the test function was never actually optimized. And the reason for that is because it's being passed objects with different hidden classes. Try changing the value of prop in a to an integer and run it again. You should see that the function was able to be optimized.

Print deoptimization stats

Modify the contents of test.js:

function test( obj ) {
  return obj.prop + obj.prop;
}

var a = { prop: 'a' }, b = { prop: [] }, i = 0;

while ( i++ < 10000 ) {
  test( i !== 8000 ? a : b );
}

Run d8 --trace-opt --trace-deopt test.js

You should see that the optimized code for the test function was thrown out. What happened here was that V8 kept seeing test being passed an object that looked like {prop: <String>}. But on the 8000th round of the while loop, we gave it something different. So V8 had to throw away the optimized code, because its initial assumptions were wrong.

Profiling

Modify test.js:

function factorial( n ) {
  return n === 1 ? n : n * factorial( --n );
}

var i = 0;

while ( i++ < 1e7 ) {
  factorial( 10 );
}

Run time d8 --prof test.js (Generates v8.log)

Run tick-processor (Reads v8.log and cats the parsed output)

This'll show you where the program was spending most of its time, by function. Most of it should be under LazyCompile: *factorial test.js:1:19. The asterisk before the function name means that it was optimized.

Make a note of the execution time that was logged to the terminal. Now try modifying the code to this dumb, contrived example:

function factorial( n ) {
  return equal( n, 1 ) ? n : multiply( n, factorial( --n ) );
}

function multiply( x, y ) {
  return x * y;
}

function equal( a, b ) {
  return a === b;
}

var i = 0;

while ( i++ < 1e7 ) {
  factorial( 10 );
}

Run time d8 --prof test.js

Run tick-processor

Roughly the same execution time as the last function, which seems like it should have been faster. You'll also notice that the multiply and equal functions are nowhere on the list. Weird, right?

Run d8 --trace-inlining test.js

Okay. So, we can see that the optimizing compiler was smart here and completely eliminated the overhead of calling both of those functions by inlining them into the optimized code for factorial.

The optimized code for both versions ends up being basically identical (which you can check, if you know how to read assembly, by running d8 --print-opt-code test.js).

Tracing Garbage Collection

Modify test.js

function strToArray( str ) {
  var i = 0,
    len = str.length,
    arr = new Uint16Array( str.length );
  for ( ; i < len; ++i ) {
    arr[ i ] = str.charCodeAt( i );
  }
  return arr;
}

var i = 0, str = 'V8 is the collest';

while ( i++ < 1e5 ) {
  strToArray( str );
}

Run d8 --trace-gc test.js

You'll see a bunch of Scavenge... [allocation failure].

Basically, V8's GC heap has different "spaces". Most objects are allocated in the "new space". It's super cheap to allocate here, but it's also pretty small (usually somewhere between 1 and 8 MB). Once that space gets filled up, the GC does a "scavenge".

Scavenging is the fast part of V8 garbage collection. Usually somewhere between 1 and 5ms from what I've seen -- so it might not necessarily cause a noticeable GC pause.

Scavenges can only be kicked off by allocations. If the "new space" never gets filled up, the GC never needs to reclaim space by scavenging.

Modify test.js:

function strToArray( str, bufferView ) {
  var i = 0,
    len = str.length;
  for ( ; i < len; ++i ) {
    bufferView[ i ] = str.charCodeAt( i );
  }
  return bufferView;
}

var i = 0,
  str = 'V8 is the coolest',
  buffer = new ArrayBuffer( str.length * 2 ),
  bufferView = new Uint16Array( buffer );

while ( i++ < 1e5 ) {
  strToArray( str, bufferView );
}

Here, we use a preallocated ArrayBuffer and an associated ArrayBufferView (in this case a Uint16Array) in order to avoid reallocating a new object every time we run strToArray(). The result is that we're hardly allocating anything.

Run d8 --trace-gc test.js

Nothing. We never filled up the "new space", so we never had to scavenge.

One more thing to try in test.js:

function strToArray( str ) {
  var i = 0,
    len = str.length,
    arr = new Uint16Array( str.length );
  for ( ; i < len; ++i ) {
    arr[ i ] = str.charCodeAt( i );
  }
  return arr;
}

var i = 0, str = 'V8 is the coolest', arr = [];

while ( i++ < 1e6 ) {
  strToArray( str );
  if ( i % 100000 === 0 ) {
    // save a long-term reference to a random, huge object
    arr.push( new Uint16Array( 100000000 ) );
    // release references about 5% of the time
    Math.random() > 0.95 && ( arr.length = 0 );
  }
}

Run d8 --trace-gc test.js

Lots of scavenges, which is expected since we're no longer using a preallocated buffer. But there should also be a bunch of Mark-sweep lines.

Mark-sweep is the "full" GC. It gets run when the "old space" heap reaches a certain size, and it tends to take a lot longer than a scavenge. If you look at the logs, you'll probably see Scavenge at around ~1.5ms and Mark-sweep closer to 25 or 30ms.

Since the frame budget in a web app is about 16ms, you're pretty much guaranteed to drop at least 1 frame every time Mark-sweep runs.

Random stuff

d8 --help logs all available d8 flags

There's a ton there, but you can usually find what you're looking for with something like d8 --help | grep memory or whatever.

d8 --allow-natives-syntax file.js

This actually lets you call V8 internal methods from within your JS file, like this:

function factorial( n ) {
  return n === 1 ? n : factorial( --n );
}

var i = 0;

while ( i++ < 1e8 ) {
  factorial( 10 );
  // run a full Mark-sweep pass every 10MM iterations
  i % 1e7 === 0 && %CollectGarbage( null );
}

...and run d8 --allow-natives-syntax --trace-gc test.js

Native functions are prefixed with the % symbol. A (somewhat incomplete) list of native functions are listed here.

Logging

d8 doesn't have a console object (or a window object, for that matter). But you can log to the terminal using print().

Comparing Hidden Classes

This is probably my favorite one. I actually just found it.

So in V8, there's this concept of "hidden classes" (Good explanation a couple paragraphs in). You should read that article – but basically, hidden classes are how V8 (SpiderMonkey and JavaScript Core use similar techniques, too) determine whether or not two objects have the same "shape".

All things considered, you always want to pass objects of the same hidden class as arguments to functions.

Anyway, you can actually compare the hidden classes of two objects:

function Class( val ) {
  this.prop = val;
}

var a = new Class('foo');
var b = new Class('bar');

print( %HaveSameMap( a, b ) );

b.prop2 = 'baz';

print( %HaveSameMap( a, b ) );

Run d8 --allow-natives-syntax test.js

You should see true, then false. By adding b.prop2 = 'baz', we modified its structure and created a new hidden class.

Node.js

A lot of these flags (but not all of them) work with Node, too. --trace-opt, --prof, --allow-natives-syntax are all supported.

That can be helpful if you want to test something that relies on another library, since you can use Node's require().

A list of supported V8 flags can be accessed with node --v8-options.

Links

Performance Tips for JavaScript in V8 (Good basic intro to Hidden Classes)

Use forensics and detective work to solve JavaScript performance mysteries

Breaking the JavaScript Speed Limit with V8

V8 - A Tale of Two Compilers (Good explanation of Inline Caches)

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Anyway, this is all still pretty new to me, and there's a lot I haven't figured out yet. But the stuff I've found so far is pretty cool, so I wanted to write something up and share it.

Oh, and I'm sure there's stuff in here that I'm wrong about, because I'm honestly a little out of my depth here. Feedback is appreciated.