Why was Dart runtime written in C++?

[Mike Lee]

Hi

I am not an expert in this field at all, and was curious about the decision tradeoffs made when choosing to implement the Dart runtime in C++. Was there any other languages considered? Or was C++ simply chosen because of previous experience with V8?

Specifically, I'm curious as to why not Go? Is a garbage collected language even feasible to be used as a runtime? I'm very curious as to what the advantages/disadvantages would be if Go was used as the runtime.

This may be a silly question but I'd appreciate any answers.

thanks! 

[Joao Pedrosa]

Hi,

C++ is one of the main blessed languages at Google. C++ is in fact used for most of Google's core software. They describe it as a single codebase that a lot of developers work on simultaneously. Google has many different developer groups. One of them is for their core server-based platforms. Another one was for the web centered around Chrome. Both of those were mostly based on C++. When they are hiring new developers, C++ must be one of their filters. With that preamble out of the way...

Dart was originally developed to be included within the Chrome browser, which is mostly C++ already. Chrome is very demanding by itself, as it is both multi-threaded and multi-process. The Dart VM they hoped would be included alongside the JavaScript VM until some day when it could be the only one. One of the difficulties of including a language VM within the browser is that garbage collection and so on needs to be shared and synchronized with the other features the browser provides like the HTML DOM. The HTML DOM is very dynamic and code and data based on it needs to be garbage collected kind of randomly. When all of that communication is happening within a single process and within a single thread, things go more smoothly than if they had to shove stuff through a bottleneck. Say, when an HTML page is refreshed, if you can free it all up by dumping all of its memory in one go it would be better than spending precious time waiting for a central database to do its job. Part of the job of garbage collection is exactly being able to free stuff up in milliseconds rather than going thru a for loop to decide what needs to be dumped. Chrome was also one of the early innovators of going multi-process rather than trying to do everything from just multi-thread alone. These days, Firefox has finally been able to go in that direction, as Firefox early on was heavily dependent on multi-threading alone. On Unix, multiple processes has been a feature for a long time. Windows went the multi-threading way. As Google was a company that relied on Unix/Linux a lot, Google showed confidence in multiple processes and the bet paid for itself on desktop version of Chrome. Lately though with mobile, multiple processes seems to have become an issue again, with Google Chrome apparently using a lot more battery than the alternatives. And that brings us another feature of C++...

With C++, you may trade off safety for raw performance. Raw performance may pay off in better graphics and also in doing more with less battery power. Garbage collection is an extra abstraction layer that may cost in terms of battery power. C++ is not known for garbage collection. Notice also that garbage collection may require more memory to hold stuff that you don't intend to collect right away. Then you collect more stuff in one go. I've seen references that with garbage collection you may need 4 times as much memory in order to be more efficient when allocating memory and collecting garbage. With mobile and IoT and so on, memory is even more precious, as third parties when developing hardware try to do more while using less memory to save money on and sell their products more popularly.

Google is not in the business of recreating the browser in a new experimental language. If anything, it looks as though Google is downsizing the browser in order to better fit it on mobile and IoT etc.

Cheers,

Joao

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[Mike Lee]

Thanks a lot Joao, answer was perfect!