[jacques_chester]

Because:

1. the JVM is an open, stable, mature, fast platform for server applications.

2. Facebook are not moving off PHP any time soon.

3. For stuff outside its written-in-C standard lib, PHP is quite slow. PHP's garbage collection, JITting etc is nowhere near as advanced as the JVM's.

4. Since porting dynamic languages to the JVM is a well-worn pathway, why not try porting PHP and see how it performs?

If Facebook can reduce their CPU and RAM requirements by just 10%, it pays for itself many times over vs the stock PHP runtime. And based on what I've seen with JRuby (going from 50Mb MRI runtimes down to 2.5Mb runtimes that are 3-10x faster) that's entirely reasonable.

[dataminer]

What kind of computations were you doing on MRI which resulted in such performance boost by switching to JRuby? I have tried Rails under JRuby many times, the performance is the same as 1.9.3 and memory consumption usually higher.

[jacques_chester]

Redmine. Runs faster and smaller per instance of the application.

[jacques_chester]

Looking at my post again, I guess I didn't do a very good job.

I didn't mean 2.5Mb per copy of the JVM. I meant per runtime within the JVM. These are run as threads, rather than processes.

So we're really comparing

    n instances of MRI * 50Mb

vs

    1 JVM + (n * 2.5Mb)

Where the cost of the JVM itself is amortised over the application instances.

Sorry, what I said originally is easily misleading.