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WSL2 is not blazingly fast.

I use Haskell for my ever continuing self-education and compiling and linking times with ghc under WSL2 is at least twice as slow as the same operations under native Linux on a much less capable machine.

It is not just me, it is acknowledged WSL2 is very slow in I/O: https://www.phoronix.com/review/windows10-okt-wsl/2

I use WSL2 for reasons but I think I will be much better off with Linux.

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If you cross the IO boundary to the Windows FS, yes, it is slow and your compilation will be slow. You need to have the project in the WSL FS, it's always been this way and I get pretty much native performance this way. Your editor can navigate to the WSL filesystem instead.

> If you cross the IO boundary to the Windows FS, yes, it is slow and your compilation will be slow

If you need good IO performance, WSL1 is still a valid option.

I need good IO performance, and it's not clear how much of the overhead is due to NTFS, and how much is due to 9p (to pass the VHDX file on the NTFS partition to WSL2)

So I'm planning some experiments with OpenZFS for windows and NTFS3 (https://old.reddit.com/r/zfs/comments/10lcnpk/zfs_raw_passth...), first using a WSL2 kernel with the option enabled (https://github.com/microsoft/WSL/issues/8564 and https://github.com/bioluks/WSL2-Linux-Kernel-NTFS) then WSL1.

I have a dual NVMe setup and W11 for Workstations, so it should be possible to run a passthrough of both partitions to the kernel.

However, it's not clear yet how to do that with a VHD on VHDX: most of the documentation I've read about VHD (or VHDX) refers to a passthrough as an "old" option: https://www.altaro.com/hyper-v/hyper-v-pass-through-disks/

Have you considered swapping Windows and Linux? Depending on your use case, you can still get pretty good performance out of virtualized Windows if you use PCI-E passthrough

I have: I generally prefer Windows, but depending on the test results, this option is definitely on the table!

As the computer has 2x NVMe and 2x GPU, it should be easy to do a GPU passthrough (with one GPU for Linux native, the other in passthough to Windows) but this brings a little extra complexity I'd rather avoid if I can.

WSL2 with code inside the Linux file system? Because if you're doing Linux work on the Windows file system, it will be bad. It is at native speeds inside the Linux file system though.

Yes, the file system appears to be Windows one.

I remember no questions being asked when I set up WSL2.

I really doubt that WSL2 will be at native speed with the Linux file system, because reads and writes still have to go through Windows kernel to be mapped into actual hardware reads/writes.

They will go through Hyper-V, not Windows

> I really doubt that WSL2 will be at native speed with the Linux file system, because reads and writes still have to go through Windows kernel to be mapped into actual hardware reads/writes.

No, Linux is running alongside Windows, not under it. Windows itself is virtualized when WSL2 is turned on, and both Linux and Windows run under Hyper-V. It's quite fancy actually.

> Windows itself is virtualized when WSL2 is turned on

Is there documentation/explainer for this? I've been trying to understand more about this (was this a change in Windows 11? Is it only when WSL2 is installed?) for a while now, but not found anything apart from stray HN comments every now and then.

It's the fundamental difference between WSL1 and WSL2. As designed in the 90s, the Windows NT kernel can have different subsystems that offer different userspace APIs to different software. Originally there were the Win32 subsystem, the Security subsystem, the Posix subsystem and the OS/2 subsystem. The latter two fizzled out, but WLS1 is basically just another subsystem that provides a linux-compatible API to the Windows kernel. As it turns out providing full compatability is really hard though, so WSL2 is a complete rewrite, ignoring the subsystem concept. It basically just runs linux in Hyper-V, and adds some UI/UX to make that convinient.

That Windows is then running virtualized is just how Hyper-V works, as soon as you turn on Hyper-V the host Windows runs as a guest in Hyper-V, though with special privileges [1]

https://learn.microsoft.com/en-us/virtualization/hyper-v-on-...

> WLS1 is basically just another subsystem that provides a linux-compatible API to the Windows kernel

WSL1 is not an NT subsystem, either. WSU (the POSIX subsystem) was an actual NT subsystem, but with WSL, they abandoned this concept and went with a different design for performance reasons.

https://news.ycombinator.com/item?id=25249262

https://github.com/microsoft/WSL/issues/2395

This is still a problem with WSL2, at least on my WSL2 with Windows file system mappings.

I tried to use instructions that use fossil from the hctree page: https://sqlite.org/hctree/doc/hctree/doc/hctree/index.html

Fossil employs sqlite to store SCM information and it fails with "SQLITE_IOERR(1290): os_unix.c:39533: (22) fsync(...)" error, which is the same with VirtualBox: https://www.fossil-scm.org/forum/forumpost/7fb6c96d80?t=c

So WSL2, having to put up with Windows quirks, is not much a Linux anymore. It is slow and some programs can't even run.

A valid critic given that there's no native filesystem support yet.

But if WSL2 is given its own drive to mount and do what it wants with it, the issue should disappear.

And when WSL2 supports passing partitions, the issue will vanish.

WSL2 is powered by Hyper-V, which is a "type 1 hypervisor":

https://en.m.wikipedia.org/wiki/Hypervisor#Classification

https://learn.microsoft.com/en-us/windows-server/administrat...

I wrote something on this topic a while ago: https://jmmv.dev/2020/11/wsl-lost-potential.html

Note that WSL 1 doesn't use the NT subsystem functionality. See this sibling comment for details https://news.ycombinator.com/item?id=34640434

> I remember no questions being asked when I set up WSL2.

If the (WSL) path to your project is inside ~/ it's on the linux file system. If it's in /mnt/c/ it's in the Windows filesystem. In WSL1 those were about equally fast, both going throught the windows kernel. With WSL2, accessing /mnt/c/ from Linux has to go through Windows, and accessing \\wsl$ from Windows has to go through Linux, so where you put the files decides where they are fast.

> In WSL1 those were about equally fast, both going throught the windows kernel

Yes, WSL1 was great. I hope it will never be sunset, as I need good IO performance.

> where you put the files decides where they are fast.

I also also hope we'll eventually get an option to pass partitions to HyperV, which is currently possible only with full disks.

It would be a return to the previous WSL1 level of performance for files within a NTFS partition: even if you'd need a separate NTFS partition, that'd be a minor nuisance to guarantee performance.

> I really doubt that WSL2 will be at native speed with the Linux file system

I had the same doubts, but when I finally took the time to install it, I found out that I was wrong. One Java program that I use, which is very heavy on the filesystem and slow like hell under Windows it a LOT faster under WSL2.

As Java profiler support is poor on Windows, I now use WSL2 to run async-profiler sessions and get results as accurate as on native Linux.

EDIT to add: you really have to run things fully in the WSL2 filesystem, be it for compiling or using a program. The performance is awful if you run WSL2 processes under the Windows filesystem.

You are incorrect in the last paragraph but that was already explained here. Try putting the files in the Linux file system and only using that.

WSL2 is actually just a Linux VM running directly on Hyper-V (not under Windows). It provides 99.5% native performance.

If you're accessing files through SMB or something then sure, it'll be slow.

I agree. If I would stay solely inside the WSL2 VM, it might be fast but my problem is that I need a tight integration between the Windows and the WSL2 file systems. In effect, this is a real pain, especially when it comes to I/O. But WSL2 also constantly loads one or more CPU cores to 100%+. My whole system feels handicapped. I probably have additional problems due to MS Defender, all known measures (that I can implement at all in a corporate-managed environment) have hardly remedied it.

Not sure if it helps in your case, but you also can access wsl disk from windows side. This could give better performance for some uses.

Having to use both msys2 and WSL2 is a bummer :(

It's probably due to hooks on NTFS file open. It shouldn't happen for files inside the ext4.vhdx (the / partition)

I wonder how VirtualBox would fare in your use case then. Especially since one can tweak I/O settings easily inside Linux too.

VMWare is a must for Windows. Code compiles 10x faster in a virtual machine on Windows than it does if you compile on Windows natively. Or 100x faster if you forgot to disable Windows Defender. I've always found that amusing.

Any idea of why this is?

Most compilation processes create a lot of very small files. Or a metric ton of them if you're unlucky enough to do js.

NTFS is awful at doing lots of small accesses on very small files, at least compared to Ext4. Add to that the fact that if you're on WSL, it needs to write POSIX attributes (which NTFS has no support for and therefore WSL must emulate), and that's a recipe for builds being slowed massively by IO.

As for Defender, well, Defender sees new files and halts everything while it scans them. You can see how that can be a problem for a process that pops out tens of thousands of temporary files, which it will all go and scan, locking them while it does so.

Another thing that makes Windows builds go slow is the lack of vfork(). On Linux, vfork() lets you create and teardown a new process in ~30 microseconds. On Windows it takes about 6000 microseconds to do the same thing, which is in large part due to the lack of copy-on-write memory (which Windows won't support, due to Microsoft corporate culture being heavily biased against overcommit models).

> which NTFS has no support for and therefore WSL must emulate

With ntfs3 (kernel 6) this is no longer true, as can be seen for example in Arch install living next to windows on C:\ https://gist.github.com/motorailgun/cc2c573f253d0893f429a165...

Also NTFS has had POSIX attributes support - they just weren't used before.

Note that I said WSL, not WSL2.

And I insist: NTFS has support for POSIX attributes. NTFS3 proves that: you can install arch on C:\, and use it normally.

WSL1 doesn't support POSIX attributes, just like ntfs-3g didn't support them, but it's not a limitation of NTFS.

That's not my experience with C++.

Full transparency, I work at Microsoft on the hypervisor.

What do you find that VirtualBox does better than Hyper-V?

I use VMWare Workstation Player to run a bunch of different operating systems on Windows, e.g. FreeBSD, OpenBSD, Windows 7, etc. It's served my needs pretty well for some time. Hyper-V as I understand it is pretty new. I think I'm using it to run WSL2 and I'm pretty happy with its performance. However I don't know how to use Hyper-V as a VMWare replacement. I'd imagine if it's able to do that, the learning curve would be a bit higher due to the CLI.

relevant bits from the comment you're replying to

> They are both impacted by storage overhead

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