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Vatsal Trivedi
Vatsal Trivedi

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My SonarQube scans were crawling; turns out Docker on WSL 2 only had 1 CPU and 1 GB of RAM

#wsl #docker #sonarqube #windows

If you're running SonarQube (or anything CPU- and memory-hungry) inside Docker Desktop on Windows and the scans feel like they're running through molasses, your .wslconfig is probably the first place to look. That was my story this week, and the fix was satisfyingly small.

Here's the whole journey, end to end, including the gotchas.

Symptom: scans stall, fans spin, nothing finishes

I was running a SonarQube scan on a mid-sized codebase inside a Docker container on Windows. The scan would chug along for a few minutes, then either crawl or stall outright. Nothing in the SonarQube logs screamed "out of memory" but the host felt fine, so it wasn't a Windows-side resource problem.

Diagnosis: ask Docker what it actually has

Docker Desktop on Windows runs everything inside a WSL 2 VM called docker-desktop. Whatever resources you've given that VM is the ceiling for every container you run. You can ask it directly:

wsl -d docker-desktop -- nproc
wsl -d docker-desktop -- free -h
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The output I got:

1
              total        used        free      shared  buff/cache   available
Mem:           907M       ...
Swap:          4.0G        ...
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One CPU. Less than 1 GB of RAM. No wonder. SonarQube's analyzer plus the Java heap alone wants more than that. Anything that hit a swap-heavy phase was going to thrash.

Fix: .wslconfig

WSL 2 reads global VM settings from a single file at %UserProfile%\.wslconfig. It doesn't exist by default you create it. Mine ended up looking like this:

[wsl2]
memory=8GB
processors=4
swap=8GB
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Three lines. That's the whole fix.

A few notes on syntax that tripped me up briefly:

  • The section header must be [wsl2] — lowercase, exact.
  • memory and swap are "size" values. You must include the unit (GB or MB). Per Microsoft's spec: "Entries with the size value default to B (bytes), and the unit is omissible." Which is a polite way of saying that swap=4 means 4 bytes, not 4 GB. Don't do what I almost did.
  • No spaces around the =.
  • processors is an integer.

Apply it

Save the file to C:\Users\<you>\.wslconfig, then in PowerShell:

wsl --shutdown
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Quit Docker Desktop from the tray icon and start it again. That's it.

The 8-second rule. Microsoft's docs are explicit about this: even after wsl --shutdown, give it a moment for the VM to fully stop before relaunching. You can verify with:

wsl --list --running
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If nothing's listed, you're safe to start back up.

Verify

After Docker came back up:

PS> wsl -d docker-desktop -- nproc
4

PS> wsl -d docker-desktop -- free -h
              total        used        free      shared  buff/cache   available
Mem:           7.8G      485.8M        6.6G        3.1M      697.7M        7.1G
Swap:          8.0G           0        8.0G
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4 CPUs, 7.8 GB RAM (the 8 GB cap, minus a sliver of overhead), 8 GB of swap. Exactly what the config asked for.

The SonarQube scan that had been crawling finished in a sensible amount of time on the next run.

Optional extras worth knowing about

If you want to go further, the [wsl2] section supports a bunch of other documented keys. The two I'd actually consider for a SonarQube-style workload:

[wsl2]
memory=8GB
processors=4
swap=8GB
vmIdleTimeout=60000

[experimental]
autoMemoryReclaim=gradual
sparseVhd=true
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  • vmIdleTimeout shuts the VM down after 60 s of idle, so it's not hoarding 8 GB while you're not using Docker.
  • autoMemoryReclaim=gradual returns idle memory back to Windows slowly instead of dropping caches abruptly nicer for scans that have memory spikes.
  • sparseVhd=true makes new WSL VHDs sparse, so disk usage actually shrinks when you free space inside the VM.

Gotchas I want to save you from

A few things I learned the hard way or almost did:

Don't allocate everything. Setting memory to your entire physical RAM, or processors to your full core count, will starve Windows itself. Aim for roughly 75% of physical RAM and leave a couple of cores for the host.

pageReporting isn't a real key. I saw it referenced in older blog posts. It's not in the current Microsoft spec - WSL silently ignores unknown keys, so the file looks valid but the setting does nothing. Stick to the documented list.

Path values need escaped backslashes. If you set swapFile or kernel, write it as C:\\Temp\\swap.vhdx, not C:\Temp\swap.vhdx.

Docker Desktop's docker-desktop VM is what you care about, not a user distro. When validating, run wsl -d docker-desktop -- free -h, not wsl -- free -h. The latter checks whatever default distro you have installed (Ubuntu or similar), which is a separate VM.

TL;DR

If Docker on Windows feels sluggish, check what WSL 2 is actually giving it:

wsl -d docker-desktop -- nproc
wsl -d docker-desktop -- free -h
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If those numbers are tiny, drop a .wslconfig at %UserProfile%\.wslconfig:

[wsl2]
memory=8GB
processors=4
swap=8GB
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wsl --shutdown, restart Docker Desktop, verify, and get on with your day.

Reference: Microsoft's advanced settings configuration in WSL.

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