[LRUG] Server capacity

Alex Tomlins alex.tomlins at unboxedconsulting.com
Tue Mar 13 07:13:37 PDT 2012

On 12/03/2012 16:48, Matthew Rudy Jacobs wrote:
> On 13 March 2012 00:11, Andrew Stewart <boss at airbladesoftware.com 
> <mailto:boss at airbladesoftware.com>> wrote:
>     Hola El Rug,
>     What sort of capacity do people run their servers at?
>     I have a Rails app on a 1-core Ubuntu 10.04 VPS with 1GB RAM.
>      Everything runs on that box: web server (Nginx), app servers
>     (Unicorn with 5 workers), MySQL, background jobs etc etc.
> Passenger suggests a 256mb VPS also running MySQL should be able to 
> run 2 passenger processes.
> By that logic a 1gb VPS should be able to run 8 passenger processes.
They also recommend running 30 on a 2Gig VPS 

Having said that, I've found that in practice the number of workers you 
can run is more constrained by the number of CPU cores available.  You 
reach a point where adding more workers just increases the CPU 
contention, and the internal response times just end up increasing, so 
the net gain is zero (or even negative).  In the past, I found that the 
sweet spot for a particular (old Rails 2.1) app was around 1.5 workers 
per CPU core.  Obviously this will vary with the app, and the hardware, 
but it's worth experimenting with various numbers to find the sweet spot 
before the Rails response times start climbing significantly.

Also, bear in mind that any "free" memory is actually used for disk 
cache, so you want to make sure that there's a reasonable amount available.


Alex Tomlins 	

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