id = 2906
title = "x86 (32-bit) multicore very slow, but x86-64 is fast (on macOS arm64 host)"
state = "opened"
created_at = "2025-04-01T17:28:21.969Z"
closed_at = "n/a"
labels = ["accel: TCG"]
url = "https://gitlab.com/qemu-project/qemu/-/issues/2906"
host-os = "- OS/kernel version: Darwin laptop.local 24.4.0 Darwin Kernel Version 24.4.0: Wed Mar 19 21:16:34 PDT 2025; root:xnu-11417.101.15~1 RELEASE_ARM64_T6000 arm64"
host-arch = "arm64"
qemu-version = "9.2.3"
guest-os = "Windows XP, 7, 10, etc."
guest-arch = "x86"
description = """More cores doesn't slow down a x86-32 guest on an x86-64 host, nor does it slow down an x86-64 guest on an arm64 host. However, adding extra cores massively slows down an x86-32 guest on an arm64 host."""
reproduce = """1. Run 32-bit guest or 32-bit installer
2.
3.

I have replicated this over several OSes using homebrew qemu, source-built qemu and UTM. This is not to be confused with a different bug in UTM that caused its version of QEMU to be slow.

This also seems to apply to 32-bit processes in an x86-64 guest."""
additional = """https://github.com/utmapp/UTM/issues/5468"""