* [Bug 207357] New: Paravirtualized spinlocks causing very severe performance degradation with 3rd gen AMD Threadripper
@ 2020-04-19 17:08 bugzilla-daemon
2020-04-19 17:12 ` [Bug 207357] " bugzilla-daemon
0 siblings, 1 reply; 2+ messages in thread
From: bugzilla-daemon @ 2020-04-19 17:08 UTC (permalink / raw)
To: kvm
https://bugzilla.kernel.org/show_bug.cgi?id=207357
Bug ID: 207357
Summary: Paravirtualized spinlocks causing very severe
performance degradation with 3rd gen AMD Threadripper
Product: Virtualization
Version: unspecified
Kernel Version: 5.6.5
Hardware: x86-64
OS: Linux
Tree: Mainline
Status: NEW
Severity: normal
Priority: P1
Component: kvm
Assignee: virtualization_kvm@kernel-bugs.osdl.org
Reporter: abj0000@gmail.com
Regression: No
I've noticed very severe performance degradation on one of the few benchmarks I
have been running, and narrowed it down the paravirtualized spinlocks.
Host:
- Fedora 32 with kernel 5.6.5 / Proxmox 6.1-8 (tried both)
- Qemu 4.1 / 4.2 / 5.0.0 rc3 (tried all of these)
- AMD Threadripper 3970X (Zen 2) CPU, 32C/64T
- 64GB of RAM, mostly allocated to 1GB huge pages.
Guest:
- Fedora 32 with kernel 5.6.5
The VM has the following key specs (through libvirt):
<memory unit="GiB">16</memory>
<memoryBacking>
<hugepages/>
</memoryBacking>
<vcpu placement="static">48</vcpu>
<features>
<acpi/>
<apic/>
<vmport state="off"/>
</features>
<cpu mode="host-passthrough" check="none">
<topology sockets="1" dies="1" cores="24" threads="2"/>
<feature policy="require" name="topoext"/>
</cpu>
I'm passing through an NVMe drive and running a RocksDB benchmark on it.
Bare metal scores about 1 000 000 Op/s, while the guest scores about 180 000,
i.e only 18% of bare metal. CPU usage on the host is about 900%.
CPUID on 0x40000001: eax=0x01003afb
If I add <pvspinlock state="off"/> or <hint-dedicated state="on"/>, then the
guest scores very close to bare metal in the benchmark, with CPU usage on the
host at 4700%, which means that it's successfully using resources at its
disposal.
If I understand correctly, adding the above zeroes the KVM_FEATURE_PV_UNHALT
bit in the guest's CPUID, and therefore deactivates the paravirtualized
spinlocks.
CPUID on 0x40000001: eax=0x01003a7b
If it's of any help, I also noticed further performance degradation doing the
following:
- Pinning the vCPUs:
<cputune>
<vcpupin vcpu="0" cpuset="8"/>
<vcpupin vcpu="1" cpuset="40"/>
<vcpupin vcpu="2" cpuset="9"/>
<vcpupin vcpu="3" cpuset="41"/>
...
<vcpupin vcpu="46" cpuset="31"/>
<vcpupin vcpu="47" cpuset="63"/>
</cputune>
- Adding <cache mode="passthrough" />
Enabling AVIC (AMD's equivalent to APICv) on kvm_amd and disabling x2apic and
nested virtualization to make it work did not help restore performance.
For comparison, I also tested VMware's ESXi 7.0 and out of the box got a very
high percentage of bare metal performance on that benchmark. That led me to
think it could be a bug in KVM.
Disclaimer: I'm just a simple enthusiast who doesn't even understand spinlocks,
TLB flushes, VM exists, IPI, APIC, etc. But I'm available to run commands and
provide you with their outputs.
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* [Bug 207357] Paravirtualized spinlocks causing very severe performance degradation with 3rd gen AMD Threadripper
2020-04-19 17:08 [Bug 207357] New: Paravirtualized spinlocks causing very severe performance degradation with 3rd gen AMD Threadripper bugzilla-daemon
@ 2020-04-19 17:12 ` bugzilla-daemon
0 siblings, 0 replies; 2+ messages in thread
From: bugzilla-daemon @ 2020-04-19 17:12 UTC (permalink / raw)
To: kvm
https://bugzilla.kernel.org/show_bug.cgi?id=207357
--- Comment #1 from ABJ (abj0000@gmail.com) ---
I forgot to mention that the benchmark is RocksDB, compiled and run through
Phoronix Test Suite.
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