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What is the main reason that made Facebook need to use C++ beside PHP? I am wondering if I make a website with alot of vistors would I need to use C++ as well?

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Possibly for back-end processes but you'd be crazy to do regular web development with it. –  ChaosPandion Mar 5 '11 at 18:16
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The question programmers.stackexchange.com/questions/53624/… also discusses this issue. You might want to take a look at it. –  Vitor Mar 5 '11 at 18:22
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Why does Google write their servers in C++? –  Job Mar 6 '11 at 19:09
    
@Job and Java and Python. –  rightfold Dec 11 '11 at 18:03
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@WTP, to my knowledge Python is used for the purposes of prototyping new features and testing them out. Once the user load becomes high and things need to work as fast as they possibly can, Python code gets replaced with C++ code. I am pretty sure that speed difference as small as 1.2 is a big deal for servers that face hundreds of millions of users. –  Job Dec 11 '11 at 18:23
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4 Answers

Here's why: HipHop for PHP: Move Fast :

One of the key values at Facebook is to move fast. For the past six years, we have been able to accomplish a lot thanks to rapid pace of development that PHP offers. As a programming language, PHP is simple. Simple to learn, simple to write, simple to read, and simple to debug. We are able to get new engineers ramped up at Facebook a lot faster with PHP than with other languages, which allows us to innovate faster.

Today I'm excited to share the project a small team of amazing people and I have been working on for the past two years; HipHop for PHP. With HipHop we've reduced the CPU usage on our Web servers on average by about fifty percent, depending on the page. Less CPU means fewer servers, which means less overhead. This project has had a tremendous impact on Facebook. We feel the Web at large can benefit from HipHop, so we are releasing it as open source this evening in hope that it brings a new focus toward scaling large complex websites with PHP. While HipHop has shown us incredible results, it's certainly not complete and you should be comfortable with beta software before trying it out.

HipHop for PHP isn't technically a compiler itself. Rather it is a source code transformer. HipHop programmatically transforms your PHP source code into highly optimized C++ and then uses g++ to compile it. HipHop executes the source code in a semantically equivalent manner and sacrifices some rarely used features — such as eval() — in exchange for improved performance. HipHop includes a code transformer, a reimplementation of PHP's runtime system, and a rewrite of many common PHP Extensions to take advantage of these performance optimizations.

Scaling PHP as a Scripting Language

PHP's roots are those of a scripting language, like Perl, Python, and Ruby, all of which have major benefits in terms of programmer productivity and the ability to iterate quickly on products. This is compared to more traditional compiled languages like C++ and interpreted languages like Java. On the other hand, scripting languages are known to generally be less efficient when it comes to CPU and memory usage. Because of this, it's been challenging to scale Facebook to over 400 billion PHP-based page views every month.

One common way to address these inefficiencies is to rewrite the more complex parts of your PHP application directly in C++ as PHP Extensions. This largely transforms PHP into a glue language between your front end HTML and application logic in C++. From a technical perspective this works well, but drastically reduces the number of engineers who are able to work on your entire application. Learning C++ is only the first step to writing PHP Extensions, the second is understanding the Zend APIs. Given that our engineering team is relatively small — there are over one million users to every engineer — we can't afford to make parts of our codebase less accessible than others.

Scaling Facebook is particularly challenging because almost every page view is a logged-in user with a customized experience. When you view your home page we need to look up all of your friends, query their most relevant updates (from a custom service we've built called Multifeed), filter the results based on your privacy settings, then fill out the stories with comments, photos, likes, and all the rich data that people love about Facebook. All of this in just under a second. HipHop allows us to write the logic that does the final page assembly in PHP and iterate it quickly while relying on custom back-end services in C++, Erlang, Java, or Python to service the News Feed, search, Chat, and other core parts of the site.

Since 2007 we've thought about a few different ways to solve these problems and have even tried implementing a few of them. The common suggestion is to just rewrite Facebook in another language, but given the complexity and speed of development of the site this would take some time to accomplish. We've rewritten aspects of the Zend Engine — PHP's internals — and contributed those patches back into the PHP project, but ultimately haven't seen the sort of performance increases that are needed. HipHop's benefits are nearly transparent to our development speed.

Hacking Up HipHop

One night at a Hackathon a few years ago (see Prime Time Hack), I started my first piece of code transforming PHP into C++. The languages are fairly similar syntactically and C++ drastically outperforms PHP when it comes to both CPU and memory usage. Even PHP itself is written in C. We knew that it was impossible to successfully rewrite an entire codebase of this size by hand, but wondered what would happen if we built a system to do it programmatically.

Finding new ways to improve PHP performance isn't a new concept. At run time the Zend Engine turns your PHP source into opcodes which are then run through the Zend Virtual Machine. Open source projects such as APC and eAccelerator cache this output and are used by the majority of PHP powered websites. There's also Zend Server, a commercial product which makes PHP faster via opcode optimization and caching. Instead, we were thinking about transforming PHP source directly into C++ which can then be turned into native machine code. Even compiling PHP isn't a new idea, open source projects like Roadsend and phc compile PHP to C, Quercus compiles PHP to Java, and Phalanger compiles PHP to .Net.

Needless to say, it took longer than that single Hackathon. Eight months later, I had enough code to demonstrate it is indeed possible to run faster with compiled code. We quickly added Iain Proctor and Minghui Yang to the team to speed up the pace of the project. We spent the next ten months finishing up all the coding and the following six months testing on production servers. We are proud to say that at this point, we are serving over 90% of our Web traffic using HipHop, all only six months after deployment.

How HipHop Works

The main challenge of the project was bridging the gap between PHP and C++. PHP is a scripting language with dynamic, weak typing. C++ is a compiled language with static typing. While PHP allows you to write magical dynamic features, most PHP is relatively straightforward. It's more likely that you see if (...) {...} else {..} than it is to see function foo($x) { include $x; }. This is where we gain in performance. Whenever possible our generated code uses static binding for functions and variables. We also use type inference to pick the most specific type possible for our variables and thus save memory.

The transformation process includes three main steps:

  1. Static analysis where we collect information on who declares what and dependencies,
  2. Type inference where we choose the most specific type between C++ scalars, String, Array, classes, Object, and Variant, and
  3. Code generation which for the most part is a direct correspondence from PHP statements and expressions to C++ statements and expressions.

We have also developed HPHPi, which is an experimental interpreter designed for development. When using HPHPi you don't need to compile your PHP source code before running it. It's helped us catch bugs in HipHop itself and provides engineers a way to use HipHop without changing how they write PHP.

Overall HipHop allows us to keep the best aspects of PHP while taking advantage of the performance benefits of C++. In total, we have written over 300,000 lines of code and more than 5,000 unit tests.

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This is the answer, a little color would have been nice, but basically it comes down to performance. They want their application to scale better, but they also have more skilled people in PHP and feel PHP makes them more productive. So the solution allows the simplicity of PHP with the performance of C. Their code base is too large to manually translate it anyway. –  Cervo Mar 6 '11 at 18:01
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Short answer -- no, you don't need C++.

I believe you're referring to HipHop. If you read about the improvements it gives, it's about 50%. This is very important if you're Facebook which has thousands of frontend servers. Cutting load on them means millions in data center cost savings. In case of company that has 10-100 frontend servers, the cost of development would much out-weight possible savings. Anyway, the most heavy processing is done in the back-end, which is usually some RDBMS or NoSQL solution, developed as highly optimized C/C++ code.

Of course the could have achieved much higher savings just dumping PHP completely. But that's not something you can do with huge code base, as in case of Facebook.

Now, for other part of the question: if you really want to know how to deal with huge loads, read the High Scalability blog, especially the Real Life Architectures part.

It is possible with PHP, but definitely wouldn't be my choice. If you want dynamic language, Python, Ruby or maybe Lua would be much better choice.

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+1, but scalability has practically nothing to do with the language used. The database architecture has a much bigger bearing. –  dan_waterworth Mar 5 '11 at 18:41
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@Dan: that's why I'm saying -- it is possible with PHP. –  vartec Mar 5 '11 at 18:44
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According to the blog entry @Trinidad cited, 50% is the average improvement, not the maximum. –  Jerry Coffin Mar 5 '11 at 21:21
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@Jerry: right, but it still very, very long way off from actual performance difference between C++ and PHP. –  vartec Mar 6 '11 at 17:08
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The nice thing about FB's approach is that they didn't have to decide right away. You should do the same. Choose the language that makes you most productive, but be sure it interfaces easily with C/C++.

Once you have millions of users and need to shave milliseconds off response times, you can optimize critical paths using C/C++.

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The C++ element used by Facebook is the HHVM.

That is the HIPHOP virtual machine. Although written in C++ it is essentially a "better PHP interpreter".

The php code is compiled into byte code which is then interpreted by the HHVM at execution time and is subject to "Just In Time" optimizations as it runs.

The developers at face book still write mostly in PHP, and, what you see on your browser is almost certainly output by a PHP program -- allbeit on which has been compiled and interpreted by the HHVM runtime.

The actual HHVM is freely available open source. So if you do need to serve one million cute kitten pictures per hour then you can download the HHVM and speed up your server with minimal change to your PHP.

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