PhantomJS with embedded web server uses only one CPU

Question

I have a problem using PhantomJS with web server module in a multi-threaded way, with concurrent requests.
I am using PhantomJS 2.0 to create highstock graphs on the server-side with Java, as explained here (and the code here). It works well, and when testing graphs of several sizes, I got results that are pretty consistent, about 0.4 seconds to create a graph.

The code that I linked to was originally published by the highcharts team, and it is also used in their export server at http://export.highcharts.com/. In order to support concurrent requests, it keeps a pool of spawned PhantomJS processes, and basically its model is one phantomjs instance per concurrent request.

I saw that the webserver module supports up to 10 concurrent requests (explained here), so I thought I can tap on that to keep a lesser number of PhantomJS processes in my pool. However, when I tried to utilize more threads, I experienced a linear slow down, as if PhantomJS was using only one CPU. This slow-down is shown as follows (for a single PhantomJS instance):

1 client thread, average request time 0.44 seconds.
2 client threads, average request time 0.76 seconds.
4 client threads, average request time 1.5 seconds.

Is this a known limitation of PhantomJS? Is there a way around it?

(question also posted here)

Answer 1

Is this a known limitation of PhantomJS?

Yes, it is an expected limitation, because PhantomJS uses the same WebKit engine for everything and since JavaScript is single-threaded, this effectively means that every request will be handled one after the other (possibly interlocked), but never at the same time. The average overall time will increase linearly with each client.

The documentation says:

There is currently a limit of 10 concurrent requests; any other requests will be queued up.

There is a difference between the notions of concurrent and parallel requests. Concurrent simply means that the tasks finish non-deterministically. It doesn't mean that the instructions that the tasks are made of are executed in parallel on different (virtual) cores.

Is there a way around it?

Other than running your server tasks through child_process, no. The way JavaScript supports multi-threading is by using Web Workers, but a worker is sandboxed and has no access to require and therefore cannot create pages to do stuff.