6 ms·

Understanding V8

75 points by philipDS 15y ago

pornel 15y ago
In hot code Avoid const I wonder why?
- mraleph 15y ago
  Because: 1) const is not supported by an optimized compiler and any function that declares or references const variable will not be optimized; 2) const is slower than var because it has a more complicated semantics;
ldite 15y ago
For those like me, who initially assumed that this didn't work in their browser for some stupid reason, turns out you need to press the space bar or page up/down.
- eis 15y ago
  You can also click the sides (left or right) of the page. Not very intuitive interface, got me too :)
  - pavlov 15y ago
    I don't understand why anyone would deliberately make a website with such simple navigation completely hidden.
    - mraleph 15y ago
      I am very sorry about unintuitive interface, but these are slides for the talk I gave on nodecamp.eu, not a website. I'll add a note on the first slide as soon as I get home.
      - pavlov 15y ago
        Ah, I see. When using the same website both for the actual presentation and for disseminating the slides afterwards, it might make sense to have some kind of navigation visible by default. You could then have a "presentation mode" shortcut key that hides the nav element when you're showing the slides on the big screen. This way, the same site would serve both audiences.
sylvinus 15y ago
One of the best presentations of nodecamp.eu
andypants 15y ago
Awesome! Is there a video of the presentation somewhere, or a transcript? I've love to hear/see the talk.
- mraleph 15y ago
  Unfortunately the talk was not recorded. There is no transcript either.
  - devinus 15y ago
    Will there be a transcript available in the future?
    - mraleph 15y ago
      Probably not. Everything is there in the deck. If people will have a lot of questions about slides (there is nothing novel there though) I can do a follow up post.
      - darklajid 15y ago
        I'd be glad if you could do that. Might be a lot to ask, depending on the time needed to write a couple of lines - but I guess right now it's most helpful for the people that are already educated on v8 performance. It's just that you could easily educate a broader audience (cough - like the author here) with a short write up. Some things are 'obvious' (GC pressure), some (everything about 'In hot code', some of the trace parameters) things are opaque and ~meaningless~.
        kragen 15y ago
        "In hot code" means "in code that comprises a substantial part of your execution". Presumably if you run try-catch or eval in non-hot code, it still runs slow, but not slow enough to make your whole program slow. I wondered too if they had more to say about the usefulness of some of the trace parameters. I am pretty sure they refer to what the profiler traces (and logs), but I don't know whether they got any usefulness out of them.
kragen 15y ago
It looks like a fantastic presentation. There were a lot of things I didn't understand from just the slides though.
- mraleph 15y ago
  Just ask if you are interested in something.
  - kragen 15y ago
    I was reluctant to ask because all of these are potentially questions that I could answer myself with sufficient effort. What's the relevance of the "GC cost" slide to the two code-change slides that follow it? Does manually converting tail-recursion have a significant impact on the number of long-lived objects? Are you using cons-strings somewhere? (I was thinking of something like Erlang IO lists. But maybe you just mean the for (...) { x.push(s); } return x.join('') trick?) Why is "indexing" relevant in "don't mix indexing and concatenation"? It looks to me like the problem is building up a big string by repeated concatenation, no? Not indexing. What's ~ToNumber? Is that a destructor? Or is it just the conversion applied by Number()? What did you use the typed arrays for? Where do you get HiddenClass from? It's not in the global namespace of my copy of node.js by default. Did you have an example where you were able to convert a polymorphic call site to monomorphic? Also, does V8 use PICs at all, or does it just have a monomorphic inline cache like in Deutsch & Schiffman? What's the connection between dictionary mode and the code on the dictionary-mode slide? What causes the arguments object to materialize? On "Two types of variables", it appears that retaining g will result in v3 not being garbage collected, potentially resulting in retaining an arbitrarily large amount of garbage. Does that really happen?
    - mraleph 15y ago
      1. V8 does not perform tail call elimination (AFAIK none of the major JS VMs perform it) because certain legacy JS language features (namely func.arguments and similar) are not TCE-friendly (see for more details http://code.google.com/p/v8/issues/detail?id=457 http://code.google.com/p/v8/issues/detail?id=457). So manually performing TCE indeed reduces number of (relatively) long lived objects: tails of the string created with substr. 2. I am not sure about this question. V8 uses cons-strings internally: so it is important to understand that c = a + b does not result (at least not immediately) in an allocation of a new sequential string and copying characters from a and b to it. That means that concatenating many strings does not cost much (especially if they are large) at the moment of concatenation. Actual concatenation happens later, when you start indexing into this string: V8 will flatten it (convert from cons-string form into sequential form). 3. See 2. If you intermix indexing and concatenation you basically force s to oscillate between cons and sequential forms: cons-string is created, then flattened on indexing, and result is concatenated with the string forming a cons-string which gets flattened... Resulting performance is bad because cons-strings actually become pure overhead (you waste more on their creation than you benefit from them) in this cycle of concatenation and flattening. 4. '*' as a prefix means optimized function and '~' as a prefix means non-optimized function. 5. Everything starting from 'Understanding Numbers' slide is a collection of facts unrelated to the first part of the talk about profiling. To get the idea of how typed arrays affect performance you can read my blog post: http://blog.mrale.ph/post/5436474765/dangers-of-cross-language-benchmark-games http://blog.mrale.ph/post/5436474765/dangers-of-cross-langua... 6. HiddenClass slides try to illustrate the idea of inline caching (http://en.wikipedia.org/wiki/Inline_caching http://en.wikipedia.org/wiki/Inline_caching) without using assembly. It's pseudo-code. HiddenClass is a structure that describes layout of an object. Slides #42-#50 illustrate how VM builds hidden classes hierarchy while executing the code. Slides #51-#52 try to illustrate how you can utilize hidden classes when you compile code. There are couple presentations that discuss hidden classes at the lower and more detailed level: for example Mads Ager's talk at Google IO 2009: http://youtu.be/FrufJFBSoQY http://youtu.be/FrufJFBSoQY 7. No, there were no example (it's impossible to fit that much into a single 40m talk). In V8 ICs have a special megamorphic state in addition to monomorphic state. Megamorphic stubs are not specialized for types IC had seen (unlike monomorphic stubs), instead they rely on a cache of monomorphic stubs (indexed by hidden class of the receiver). 8. Those are functions that force object from fast-mode to dictionary-mode. 9. In non optimized code it is materialized when used in the code. In optimized it is never materialized (but as the slide says: if you "misuse" arguments in a function it will not be optimized). 10. I have not seen any real world code that suffers from this problem. But people ask about scopes and closed-variables quite often that is why it is included.

Understanding V8

75 points by philipDS 15y ago

pornel 15y ago

In hot code Avoid const I wonder why?

mraleph 15y ago
Because: 1) const is not supported by an optimized compiler and any function that declares or references const variable will not be optimized; 2) const is slower than var because it has a more complicated semantics;

ldite 15y ago

For those like me, who initially assumed that this didn't work in their browser for some stupid reason, turns out you need to press the space bar or page up/down.

eis 15y ago
You can also click the sides (left or right) of the page. Not very intuitive interface, got me too :)
- pavlov 15y ago
  I don't understand why anyone would deliberately make a website with such simple navigation completely hidden.
  - mraleph 15y ago
    I am very sorry about unintuitive interface, but these are slides for the talk I gave on nodecamp.eu, not a website. I'll add a note on the first slide as soon as I get home.
    - pavlov 15y ago
      Ah, I see. When using the same website both for the actual presentation and for disseminating the slides afterwards, it might make sense to have some kind of navigation visible by default. You could then have a "presentation mode" shortcut key that hides the nav element when you're showing the slides on the big screen. This way, the same site would serve both audiences.

sylvinus 15y ago

One of the best presentations of nodecamp.eu

andypants 15y ago

Awesome! Is there a video of the presentation somewhere, or a transcript? I've love to hear/see the talk.

mraleph 15y ago
Unfortunately the talk was not recorded. There is no transcript either.
- devinus 15y ago
  Will there be a transcript available in the future?
  - mraleph 15y ago
    Probably not. Everything is there in the deck. If people will have a lot of questions about slides (there is nothing novel there though) I can do a follow up post.
    - darklajid 15y ago
      I'd be glad if you could do that. Might be a lot to ask, depending on the time needed to write a couple of lines - but I guess right now it's most helpful for the people that are already educated on v8 performance. It's just that you could easily educate a broader audience (cough - like the author here) with a short write up. Some things are 'obvious' (GC pressure), some (everything about 'In hot code', some of the trace parameters) things are opaque and ~meaningless~.
      - kragen 15y ago
        "In hot code" means "in code that comprises a substantial part of your execution". Presumably if you run try-catch or eval in non-hot code, it still runs slow, but not slow enough to make your whole program slow. I wondered too if they had more to say about the usefulness of some of the trace parameters. I am pretty sure they refer to what the profiler traces (and logs), but I don't know whether they got any usefulness out of them.

kragen 15y ago

It looks like a fantastic presentation. There were a lot of things I didn't understand from just the slides though.

mraleph 15y ago
Just ask if you are interested in something.
- kragen 15y ago
  I was reluctant to ask because all of these are potentially questions that I could answer myself with sufficient effort. What's the relevance of the "GC cost" slide to the two code-change slides that follow it? Does manually converting tail-recursion have a significant impact on the number of long-lived objects? Are you using cons-strings somewhere? (I was thinking of something like Erlang IO lists. But maybe you just mean the for (...) { x.push(s); } return x.join('') trick?) Why is "indexing" relevant in "don't mix indexing and concatenation"? It looks to me like the problem is building up a big string by repeated concatenation, no? Not indexing. What's ~ToNumber? Is that a destructor? Or is it just the conversion applied by Number()? What did you use the typed arrays for? Where do you get HiddenClass from? It's not in the global namespace of my copy of node.js by default. Did you have an example where you were able to convert a polymorphic call site to monomorphic? Also, does V8 use PICs at all, or does it just have a monomorphic inline cache like in Deutsch & Schiffman? What's the connection between dictionary mode and the code on the dictionary-mode slide? What causes the arguments object to materialize? On "Two types of variables", it appears that retaining g will result in v3 not being garbage collected, potentially resulting in retaining an arbitrarily large amount of garbage. Does that really happen?
  - mraleph 15y ago
    1. V8 does not perform tail call elimination (AFAIK none of the major JS VMs perform it) because certain legacy JS language features (namely func.arguments and similar) are not TCE-friendly (see for more details http://code.google.com/p/v8/issues/detail?id=457 http://code.google.com/p/v8/issues/detail?id=457). So manually performing TCE indeed reduces number of (relatively) long lived objects: tails of the string created with substr. 2. I am not sure about this question. V8 uses cons-strings internally: so it is important to understand that c = a + b does not result (at least not immediately) in an allocation of a new sequential string and copying characters from a and b to it. That means that concatenating many strings does not cost much (especially if they are large) at the moment of concatenation. Actual concatenation happens later, when you start indexing into this string: V8 will flatten it (convert from cons-string form into sequential form). 3. See 2. If you intermix indexing and concatenation you basically force s to oscillate between cons and sequential forms: cons-string is created, then flattened on indexing, and result is concatenated with the string forming a cons-string which gets flattened... Resulting performance is bad because cons-strings actually become pure overhead (you waste more on their creation than you benefit from them) in this cycle of concatenation and flattening. 4. '*' as a prefix means optimized function and '~' as a prefix means non-optimized function. 5. Everything starting from 'Understanding Numbers' slide is a collection of facts unrelated to the first part of the talk about profiling. To get the idea of how typed arrays affect performance you can read my blog post: http://blog.mrale.ph/post/5436474765/dangers-of-cross-language-benchmark-games http://blog.mrale.ph/post/5436474765/dangers-of-cross-langua... 6. HiddenClass slides try to illustrate the idea of inline caching (http://en.wikipedia.org/wiki/Inline_caching http://en.wikipedia.org/wiki/Inline_caching) without using assembly. It's pseudo-code. HiddenClass is a structure that describes layout of an object. Slides #42-#50 illustrate how VM builds hidden classes hierarchy while executing the code. Slides #51-#52 try to illustrate how you can utilize hidden classes when you compile code. There are couple presentations that discuss hidden classes at the lower and more detailed level: for example Mads Ager's talk at Google IO 2009: http://youtu.be/FrufJFBSoQY http://youtu.be/FrufJFBSoQY 7. No, there were no example (it's impossible to fit that much into a single 40m talk). In V8 ICs have a special megamorphic state in addition to monomorphic state. Megamorphic stubs are not specialized for types IC had seen (unlike monomorphic stubs), instead they rely on a cache of monomorphic stubs (indexed by hidden class of the receiver). 8. Those are functions that force object from fast-mode to dictionary-mode. 9. In non optimized code it is materialized when used in the code. In optimized it is never materialized (but as the slide says: if you "misuse" arguments in a function it will not be optimized). 10. I have not seen any real world code that suffers from this problem. But people ask about scopes and closed-variables quite often that is why it is included.