Does ZRAM or ZSWAP makes sense in WSL2?

[crramirez]

Windows has its own RAM compression system, and WSL2 ultimately runs inside Windows.

Does the Windows RAM manager compresses the Hyper-V RAM as well?

[crramirez]

Anyone?

[4-FLOSS-Free-Libre-Open-Source-Software]

ZRAM works very well inside WSL2. In the meantime, you could use the zRAM enabled wsl2 kernel https://github.com/Locietta/xanmod-kernel-WSL2?tab=readme-ov-file#usage

[crramirez]

Thank you for sharing. I'll test it. As I couldn't find an answer on the internet if the Hyper-V memory is compressed by the host, today after your answer I asked to ChatGPT and the following is the answer that I received:

No, the memory used by a Hyper-V virtual machine (VM) is generally not subject to compression by the host’s memory compression feature in Windows 10 and Windows 11. Here’s why:

1. Hyper-V’s Memory Allocation: Hyper-V operates at a lower level than typical user-mode applications. It uses a Type-1 hypervisor architecture, meaning the hypervisor runs directly on the hardware beneath the host operating system. The memory allocated to VMs is reserved and managed by the hypervisor, not the host OS’s memory manager.
2. Isolation from Host Memory Management: The memory used by VMs is isolated from the host’s standard memory management mechanisms. This isolation ensures that the VMs have consistent and predictable performance, which is crucial for running guest operating systems and applications reliably.
3. Memory Compression Scope: Windows memory compression is designed to work with the memory pages managed by the host’s operating system processes. It compresses the memory of user-mode applications to optimize RAM usage when the system is under memory pressure. Since the VM’s memory is managed by Hyper-V and not by the host OS’s typical memory manager, it doesn’t participate in this compression.
4. Potential Performance Impact: Compressing VM memory at the host level could introduce latency and unpredictability, negatively affecting the performance of the VMs. Hyper-V aims to provide VMs with as close to raw hardware performance as possible, so adding host-level compression would counteract this goal.
5. Dynamic Memory Feature: Hyper-V offers its own memory management feature called Dynamic Memory, which allows Hyper-V to adjust the amount of memory available to a VM based on its needs. This feature operates independently of the host’s memory compression and is designed specifically for virtualized environments.

References and Further Reading:
• Microsoft Docs - Hyper-V Dynamic Memory Overview
• Understanding Memory Compression in Windows

In summary, the host’s memory compression feature does not compress the memory used by Hyper-V virtual machines. The VM memory is managed separately by Hyper-V to ensure optimal performance and isolation.