I lost count of the number of untrue statements there after about ten.
- thought that Facebook's servers would be hosted in their office
- was unfamiliar with the idea of usability studies or tracking eyeballs during usability studies
- was baffled by the idea that facebook would track your clicks throughout the entire site to help determine who your closest contacts were
- was also surprised by the fact that in the cloud, nothing is ever truly deleted
- was surprised to see that facebook employees would have the ability to login as any user in the system
I do wish they hadn't had so many timekeeping problems, though (and I'm not sure what was up with the aggressive travelling calls in the first half either).
I guess the problem being described, somewhat obtusely, is that massive amounts of dependencies (in Facebook's case million of lines of code) are loaded in order to execute a relatively small page.
I'll admit that to a CS person who understand compilers, their description makes it look like they don know what they're talking about. But if you explained Facebook's interpretation/compilation problems to a non-techy, it would probably come out sounding similar.
(Note comment reuse: http://www.reddit.com/r/programming/comments/aodzg/one_faceb...)
SBCL and V8 and TraceMonkey are all very fast "scripting languages" because they have a good codegen in the runtime. PHP does not do any codegen.
If I were explaining this to a non-technical person, I would say: A computer works by executing instructions, one at a time. When PHP is executed, it is compiled to a list of instructions, but not ones the processor can directly understand. It uses a "virtual machine" to read each instruction and then execute the corresponding instructions on the real computer. This is slow, because what could be one instruction actually becomes many, sometimes hundreds, of actual instructions. The goal of this project is to skip the virtual machine and run our code directly on the real machine, meaning that we won't have to do as much unnecessary work. This will make our application run faster!
Now, this still might make CS folks mad, because there is bookkeeping overhead other than deciding which instructions to run (you can't store "scalars" in registers, after all), and of course modern CPUs don't execute one instruction at a time. But now we're wandering off into esoterica -- at least the basic idea is right, and I don't use meaningless expressions like "scripting language".
They're trying to explain the problems that Facebook are having, which is that there is just such a large volume of code. They aren't trying to give a short course in interpretation and compilation.
Scripting language is not a meaningless expression. Many papers use it. I submitted papers with "scripting language" in the title to PLDI and POPL, and there was not one complaint or problem with the term from reviewers (who found plenty of tiny "errors" otherwise).
By this definition C isn't a scripting language, as it was small and stood that way.
Python main() functions by Guido van Rossum
Its a tricky term to define, much like "compiler". Actually "compiler" is a nice analogy: everyone knows what one is, but the actual test for "is this a compiler" is shaky, and returns true for lots of things that aren't _really_ compilers.
Anyway, Ousterout coined it in his paper on TCL, but annoyingly chose not to formally define it. The set at that time was (I think) {TCL, Perl, sh}.
But because he didn't define it properly, the word has been used for years without a proper definition. The best I can do is to say read my PhD thesis, starting on page 7, where I'd spent a meaty six pages explaining in as much detail as I can what it means to be a scripting language.
[snip]...consequence of this is that in many implementations, operating on a variable with automatic or static lifetime through a reference, although syntactically similar to accessing it directly, can involve hidden dereference operations that are costly.[/snip]
I admit it sounds a bit off, like the quotee doesn't know exactly what she's talking about, but she's not egregiously wrong.
1) The term "scripting language"; meaningless, and usually just used as an insult (as in this case) rather than with any reasonable definition.
2) Browsers don't execute PHP
3) Not all scripting languages are executed based on line order.
4) PHP doesn't require values to be declared before they're used.
5) Not all compiled languages are compiled to a separate file.
6) Most modern compiled languages compile to byte-code, which is not more "executable" than the original source.
7) Compilers do have to read the program from beginning to end. This might just be my ignorance, but I've never heard of a random-access parser.
8) Being compiled doesn't necessarily make execution any faster.
9) The compiled file still has to be read entirely, before it can be executed. Depending on code size, sections of it may be read repeatedly.
However, not all software engineering groups have the capabilities to produce such highly optimized binaries, and there is always the risk that a user's particular usage patterns will differ enough from the expected patterns that they will lose the benefit of this extensive optimization. However, in an interpreted or byte-code language a lot of the same information needed for optimization is available to the run-time. A run-time designed for optimization may be able to take advantage of that, creating super efficient code paths based on actual usage. This model is more difficult to implement but potentially more robust than statically optimized compilation (and also has the potential to take greater advantage of differences in hardware, a statically compiled native binary doesn't have the ability to morph its optimization based on whether its running on a single core Atom or a 6-way Core i7 cpu, or some 100-core monster of the future, but a run-time potentially can).
In the average case most of this is just theory, but the potential is very real.
Some worthwhile background reading:
Trace Trees: http://www.ics.uci.edu/~franz/Site/pubs-pdf/ICS-TR-06-16.pdf
A blog post / talk from Steve Yegge on dynamic language performance and other topics: http://steve-yegge.blogspot.com/2008/05/dynamic-languages-st...
BTW these technologies are not interpreters, but compilers (but runtime compilers).
1) You can read from the interview that the quotee was speaking to a non-technical person. Even so, they're using the term in the commonly used sense. But you haven't shown that they're wrong, only that you disagree with the term.
2) You are right. This is the only falsehood I can see here. I'll stipulate that its ok in a 'you know what I meant' kinda way.
3) Your answer to 2) depends on the quotee speaking about PHP, but now they aren't? And in 1) you said that scripting languages don't exist. Basically, you're reaching. Anyway, please name a scripting language which is not executed on line order. PHP certainly is.
4) Values aren't mentioned anywhere. Anyway, PHP requires functions and classes to be declared before use. (Strictly speaking, values don't exist before they are used, so what you said doesn't make sense. I presume you meant variables, but variables aren't declared. So I presume you meant defined, but strictly speaking there is no such thing as a variable in PHP anyway, just a mapping of strings to values (see: all the literature on the topic, including mine)).
5) They're obviously talking in the context of a native compiler, so I see nothing wrong with this.
6) They do? First I heard of it. I'm guessing you've got a funny definition of 'modern compiled languages' to back this up.
7) Executables don't though.
8) Not necessarily, sure. The first iteration of my PHP compiler was 10 times slower. But to say that the quotee is _wrong_ just because some implementation can be slower than some other implementation, is faulty logic.
9) What's your point? Why are they wrong?
So I'll give you one. Two if we count number 8, but that only counts for the most extreme form of pedantry, and it doesn't really contradict anything. So, yeah, one. A long way from ten.
<?php
echo foo('baz');
function foo($bar) { return $bar; }
?>
This looks to me like you can use PHP functions before declaring them above.
echo foo('baz'); // error
{
function foo($bar) { return $bar; } // not pulled to top of file
}1) All the more reason not to introduce incorrect terminology; non-technical people won't be able to understand that it's wrong.
3) There are legitimate definitions of "scripting languages"; the one I use is that a scripting language is not useful without 3rd-party code. The Bourne shell and JavaScript are classic examples of scripting languages.
QuakeC is a scripting language which isn't executed in line order.
4) Values are procedures (PHP doesn't have functions; the keyword is mis-named), classes, or variables. They do not have to be declared before use in PHP -- if they did, writing mutually-recursive procedures would be impossible in PHP.
5) I see nothing "obvious" about your statement. They state that compiling results in a separate file, which is wrong because some compilers don't.
6) JavaScript (in all popular implementations), Python, JVM languages (Java, Scala), .NET languages (C#, F#, VB.NET), Perl 6. I think even Ruby has a bytecode compiler, now.
7) There is no indication that this new PHP implementation compiles to native executables.
8) Their claim is that compilation makes execution faster. There exist cases where compilation does not make execution faster. Therefore, their claim is incorrect.
9) Their quote states that compiled binaries don't have to be read from beginning to end to execute. This is incorrect. When a binary is executed, or a library linked, it is loaded entirely into memory.
3) Please define it. Note that I spend six pages in my PhD on defining it, and there are no formal definitions. Ousterout introduced the term, and didn't define it.
That's a crap definition of scripting language. I don't even know what it means, and it's unusable. C++ is pretty much useless without the C++ standard library. Is it a scripting language now?
3) Nice. The fact that you could dredge up a minor language which peaked in 1996 does not make the quotee wrong.
4) This is all wrong.
> They do not have to be declared before use in PHP -- if they did, writing mutually-recursive procedures would be impossible in PHP.
Whether or not mutually recursive procedures have to be declared is a matter of parsing style. For example, its there in C since it was created using a one-pass compiler. Its not there in Java. In PHP, declaring a function (say x) puts the function into the function-symbol table under the entry "x". When calling x(), the function-table is looked up. It is not necessary to have defined x to parse code that uses it. For example:
if (false) { x(); } // legit, x() is never called
> Values are procedures, classes, or variables.
I presume you mean that variables, classes and procedures are all kinds of values. That is simply incorrect. Classes are not first-class values, and first-class classes and functions are approximated in PHP by allowing classes to be instantiated by name (using a string) at run-time. This is changing slightly in 5.3, but the semantics are complicated, and still use strings.
Variables are just syntax in PHP. As I said, look at any literature on PHP (or javascript if you prefer). Variables are syntax for keys in a local symbol-table, and aren't real entities. They are certainly not values.
> PHP doesn't have functions; the keyword is mis-named.
Evidence? I can't think how this is correct.
5) So what if some compilers don't. Nearly all compilers do. The level of pedantry here is astounding. Just because you can list an edge case in which they are wrong, does not make them wrong in the general case.
6) Ah, your definition of "compiled" is "bytecode compiled". What a funny circular argument.
7) Except that the quotee says it! "But with a compiled language, the program you write is compiled into an executable file."
8) "Is a rocketship faster than a bicycle?" Yes. "Aha, but if my rocket isnt moving, the bicycle is faster. Therefore I assert that rocketships are _not_ faster than bicycles". Bollox.
9) Loading from memory is not "reading" in the sense the quotee used, which was clearly parsing. I think you're being deliberately obstinate: try using the context of the article to determine what the words mean. I could argue that binaries do not have to be fully read into memory (they are loaded lazily, page-by-4k-page, by most modern OSes), but I don't want to start a discussion on it. Heaven forbid an obscure 90s OS used 8k pages.
Anyway, you're deliberately twisting the quotee's words and being pedantic, so I'm done here.
8) Being natively compiled does not make that program automatically faster. It depends on the use case (I/O vs compute bound) and it also depends on the quality and complexity of the code generator and optimizer. Furthermore, bytecode compiled languages (as opposed to native compiled) may be able to better optimize at runtime using JIT compilation due to being able to make assumptions you couldn't make at "compile time" in AOT compilation. Saying that natively compiled programs are faster than other kinds of programs is simply not always true.