> ## Documentation Index
> Fetch the complete documentation index at: https://docs.fly.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Scaling

Scaling an application is a complex subject with many variables, in this page we will focus on a number of factors that are largely universal: number of CPU cores, memory, and both the number and placement of virtual machines.

Not covered here is scaling of databases or scaling of volumes.

For information on costs, see [Fly App Pricing](/about/pricing).

## Memory

RAM is a precious commodity - both to those on Hobby plans who want to remain within or near the free allowances, and to apps that want to scale to be able to handle a large number of concurrent connections.

At some point you may find that you need more memory. There are two types: real and virtual. Real is faster, but more expensive. Virtual (swap) is slower and generally free.

Some starting point recommendations:

* 256MB is enough to run small applications with few users
* 512MB is a better place to start if you are doing image processing or have a moderate number of users.
* 1024MB is a minimum if you are running an application that uses puppeteer or otherwise makes use of chrome's rendering engine.

Once you have settled on a size, some links to get you started:

* [`fly scale memory`](/flyctl/cmd/fly_scale_memory)
* [`swap_size_mb`](/reference/configuration#swap_size_mb-option)

These options are not mutually exclusive.  A good starting point may be 512MB of RAM and 1024MB of swap.

Note: if you do elect to use swap, Node.js may not make full use of the extra memory, resulting in an `JavaScript heap out of memory` error,  You can instruct it to use more by setting an environment variable in your `fly.toml`:

```toml theme={null}
[env]
NODE_OPTIONS='--max-old-space-size=4096'
```

See the [Node.js documentation](https://nodejs.org/api/cli.html#--max-old-space-sizesize-in-megabytes) for more information.

## CPU Cores

By default, JavaScript applications (whether they be Node.js, Deno, or Bun)
run on a single operating system thread and therefore won't benefit from
running with multiple cores.

Node.js has a [cluster](https://nodejs.org/api/cluster.html) module that can
be used to address this.  See [Run a Node.js HTTP Express.js Server on Multiple CPU Cores](https://coderrocketfuel.com/article/run-a-node-js-http-express-js-server-on-multiple-cpu-cores) for an example on how to use this module.

Once your application is coded to take advantage of multiple cores,
run `fly machines list` to see a list of your machines, and then
run [`fly machine update --vm-cpus`](/flyctl/cmd/fly_machine_update) to update the number of CPUs a single machine.

## Virtual Machines

For most JS applications the key to scaling will be in creating more virtual
machines, each with enough memory and CPU cores to handle a unit of workload.
More information on that topic can be found on the
[Scale the Number of Machines](/launch/scale-count) page.