[dancer-users] single-threaded

[Warren Young]

On Nov 24, 2015, at 6:48 AM, Rick Leir <richard.leir at canadiana.ca> wrote:
> 
> The single-threaded web server only breaks down when you start getting to load levels where the server is handling at least one connection all the time, so that allowing to to handle a second or third in parallel can provide some benefit.
> 
> What if your route handler goes to disk for something (perhaps a DB access)? Your other Starman process(es) could start handling another connection in the meantime.

A DB access should be over in a matter of milliseconds in most apps.  Let’s say your heaviest query is 50 ms.  That’s short enough that you could have 200 of those per second and still do it all in a single thread.

> What if your CPU has 2 or more cores (likely)? Again, the other process could get going. 

I thought we were talking about I/O bound processes, not CPU-bound processes.

If you take a single-threaded app that is bottlenecked on the disk, adding a second thread will just cause that first thread to spend half its time waiting on the disk, too.  (The kernel will ping-pong among those threads.)

If your DB is small enough to end up RAM-backed by the kernel’s buffer cache after it’s been running for a while, that will drop DB access times by a factor of 1000 or so, if most of your workload is reading, which increases the number of clients you can serve by a similar factor.

Alternately, if your workload doesn’t exceed 1.0 full CPU cores consistently, you’ll chew up any speed increase you get from multicore waiting on synchronization locks, context switches, etc.

Let’s also not forget that I advocated running your Dancer app behind a fully-featured proxy server such as nginx or mod_proxy, which gets you a lot of concurrency all by itself.

Try this: create environments/single-threaded.yml, with the “simple" web server and session manager.  Put it behind a proxy server, then run a concurrent web benchmark against it.  I use Siege[*], and it just gave me a concurrency value of 36.60 on my app here.  

Why is it > 1.0?  Because a huge number of the URLs in my app are static assets, not dynamic, so the proxy server handled those.


[*] https://www.joedog.org/siege-home/

> Maybe you have low traffic on an Intranet. But out on the Internet, you could have bots and spiders bothering your route handlers just when you have a real client to serve.

Going multi-threaded just means you need a bigger DDoS to take the site down.

Adding cores is no way to fight off a DDoS.  To do that, you need massive bandwidth, not cores.

Look, I’m not saying that multi-core Dancer apps have no purpose.  I’m saying that until your app starts maxing out a single core, you won’t get any benefit from making it multi-thread capable.