Passenger server upgrade: Processor (CPU) Cores VS Ram?

I went through documentation of Passenger to find out how many application instances it can run with respect to hardware configuration. Documentation only talks about RAM

The optimal value depends on your system’s hardware and the server’s average load. You should experiment with different values. But generally speaking, the value should be at least equal to the number of CPUs (or CPU cores开发者_开发百科) that you have. If your system has 2 GB of RAM, then we recommend a value of 30. If your system is a Virtual Private Server (VPS) and has about 256 MB RAM, and is also running other services such as MySQL, then we recommend a value of 2.

It says minimum value can be number of CPU/CPU Cores we have. I have a VPS with one VCPU & 1GB RAM & my service provider has an option to just upgrade the RAM. I'm wondering how far I can just keep upgrading only RAM? How important it is to upgrade number of CPUs?

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Quick Answer

Depends on what resources are the bottleneck for your app.

Long answer

You'll need to factor in a few things:

1. How much CPU time does your app need?
2. How much RAM does any given instance of your app use at peak load?
3. Does your app spend a lot of time doing IO intensive tasks? (ie: db and file reads/writes, network communication)

There can be other things to factor in, but your bottlenecks will probably be one of the above. If RAM is your main bottleneck, by all means use your newly available RAM. However, if it turns out that your app is being slowed down by CPU availability or flooded IO, no amount of RAM is going to speed things up.

On the topic of CPU cores; my understanding is that the main Apache process that runs Passenger is a single threaded process. Apache spawns new threads to handle concurrency on an as-needed basis. Each additional CPU core theoretically allows you to spawn x*n threads, where x is the number of threads you can optimally run under a single CPU core and n is the number of CPU cores available to Apache.

Disclaimer: I'm not very well read on Passenger internals; though this logic usually holds true for other kinds of Apache configurations.