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+id = 2821
+title = "Emulated newer x86 chipsets are noticably slower on cpu-bound loads than \"-cpu qemu64\""
+state = "closed"
+created_at = "2025-02-19T08:56:07.163Z"
+closed_at = "2025-02-20T08:44:22.235Z"
+labels = ["accel: TCG", "target: i386"]
+url = "https://gitlab.com/qemu-project/qemu/-/issues/2821"
+host-os = "Fedora 41 x86 (issue also observed on Asahi Linux (Fedora41 based), so aarch64)"
+host-arch = "x86, ARM"
+qemu-version = "qemu-system-x86-9.1.2-3.fc41.x86_64"
+guest-os = "Fedora 41"
+guest-arch = "x86"
+description = """I noticed that "-cpu qemu64" is much faster than "-cpu max" or "-cpu Icelake-Server-noTSX" for cpu bound loads, and with more than one cpu under load."""
+reproduce = """1. Run a guest as per "qemu-system-x86_64 -cpu max [..]" command from above. Any linux distro should do.
+2. run through the setup questions if you use Fedora-Server-KVM-41-1.4.x86_64.qcow2 from the example command line above
+3. log into the guest via ssh, i.e. "ssh chris@amd64" here
+4. cd /dev/shm; wget http://archive.apache.org/dist/httpd/httpd-2.4.57.tar.bz2; wget https://fluxcoil.net/files/tmp/job_httpd_extract_cpu.sh
+6. bash ./job_httpd_extract_cpu.sh 4 300
+8. cat /tmp/counter
+
+Step 6 is executing a script which simply uses 4 parallel loops, where each loop runs "bzcat httpd-2.4.57.tar.bz2" constantly. After 300sec, the successful uncompressions over all 4 loops are summed up and stored in /tmp/counter.
+
+- result with "-cpu qemu64": 96
+- result with "-cpu max": 84
+- result with "-cpu Icelake-Server-noTSX": 44"""
+additional = """- For "-cpu Icelake-Server-noTSX" on this Thinkpad T590 I get these warnings, I think they are not relevant:
+  qemu-system-x86_64: warning: TCG doesn't support requested feature: CPUID.01H:ECX.pcid [bit 17]
+  qemu-system-x86_64: warning: TCG doesn't support requested feature: CPUID.01H:ECX.tsc-deadline [bit 24]
+  [..]
+- I also looked at Broadwell etc, and all of them seem in the same ballpark.
+  Graph over some emulated architectures: https://fluxcoil.net/files/tmp/gnuplot_cpu-performance-emulated-only.png"""