Kernel Mitigations in WSL2 leading to 30-50% worse wall time, 80% worse syscall time

[tonymet]

Windows Version

Microsoft Windows [Version 10.0.26220.8079]

WSL Version

WSL version: 2.6.3.0

Are you using WSL 1 or WSL 2?

• WSL 2
• WSL 1

Kernel Version

6.6.87.2-1

Distro Version

Debian 13.4

Other Software

• go version go1.26.1 linux/amd64

Summary

Kernel side-channel mitigations seem to affect WSL2 performance more heavily than a normal linux kernel. A normal kernel with mitigations=on will see 5-10% performance impact. With wsl2, mitigations cause 84% syscall impact, and 47% compile time (wall time) impact for a moderate golang compile.

Repro Steps

Steps to Reproduce

1. Run perf bench sched pipe -l 100000
2. Run go clean -cache && go build ./...
3. Add kernelCommandLine=quiet loglevel=0 audit=0 mitigations=off vsyscall=none to %UserProfile%.wslconfig.
4. wsl --shutdown & re-run the tests

Expected Behavior

5-10% performance difference between mitigations=on and mitigations=off

Actual Behavior

Context Switch Latency (perf bench sched pipe -l 100000)

Metric	Mitigations OFF	Mitigations ON (Default)	Regression
Latency (usecs/op)	37.51 μs	44.45 μs	+18.5%
Throughput (ops/sec)	26,659	22,493	-15.6%

go build benchmark -- 84% worse system time, 47% worse wall time

Metric	Mitigations OFF	Mitigations ON (Default)	Regression
System Time	13.51s	24.89s	+84.2%
Total (Wall) Time	18.31s	26.98s	+47.3%
CPU Efficiency (higher=better)	522%	395%	-127%

Testing Mitigations

❯ grep Vuln /sys/devices/system/cpu/vulnerabilities/*
/sys/devices/system/cpu/vulnerabilities/spec_rstack_overflow:Vulnerable
/sys/devices/system/cpu/vulnerabilities/spec_store_bypass:Vulnerable
/sys/devices/system/cpu/vulnerabilities/spectre_v1:Vulnerable: __user pointer sanitization and usercopy barriers only; no swapgs barriers
/sys/devices/system/cpu/vulnerabilities/spectre_v2:Vulnerable; IBPB: disabled; STIBP: disabled; PBRSB-eIBRS: Not affected; BHI: Not affected

[github-actions]

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[tonymet]

WslLogs-2026-03-25_09-06-43.zip

[github-actions]

Diagnostic information

.wslconfig found
	Custom kernel command line found: 'quiet loglevel=0 audit=0 mitigations=off vsyscall=none'
Detected appx version: 2.6.3.0
Found evidence of linux crash: !usr!bin!zsh (dump: C:\Users\tonymet\AppData\Local\temp\wsl-crashes\wsl-crash-1774454831-20871-_usr_bin_zsh-11.dmp)

[tonymet]

control test of other kernel options .

Result : mitigations=off is 11% faster

A: mitigations=on kernelCommandLine=quiet loglevel=0 audit=0 mitigations=ON vsyscall=none

 perf bench sched pipe -l 100000
     Total time: 4.382 [sec]

      43.828820 usecs/op
          22816 ops/sec

B: mitigations=off kernelCommandLine=quiet loglevel=0 audit=0 mitigations=off vsyscall=none

❯ perf bench sched pipe -l 100000
     Total time: 3.928 [sec]

      39.285230 usecs/op
          25454 ops/sec