** Description changed:
- To be filled - I'm just reserving the LP number for now.
+ [Impact]
+ * Currently linux-kvm derivative doesn't have CONFIG_PCI_MSI (and its
dependency options) enabled. The goal for such derivative is to be minimal and
boot as fast as possible in virtual environments, hence most config options
were dropped.
+
+ * Happens that MSI/MSI-X are the de facto drivers' standard with regards
+ to interrupts, and as such the hot path is optimized for MSIs. Boot
+ testing with that config enabled showed that we have improvements in
+ boot time (details in next section).
+
+ * Also, performance-wise MSIs are a good idea too, since it usually
+ allows multiple queues in network devices and KVM is more optimized to
+ MSIs in comparison with regular IRQs - tests (detailed in next section)
+ showed performance improvements in virtio devices with MSIs.
+
+ * Based on that findings, we are hereby enabling MSIs for the linux-kvm
+ derivatives in all series (Bionic / Focal / Groovy / Hirsute) - notice
+ that Xenial already has that config option enabled.
+
+ [Test Case]
+ * All below tests were performed in a x86-64 KVM guest with 2 VCPUs and 2GB
of RAM, running in a Focal host. Three runs of each test were performed, and we
took the average.
+
+ * Boot time test (measured by dmesg timestamp) showed an improvement of ~21%,
the following chart exhibiting the data:
https://kernel.ubuntu.com/~gpiccoli/MSI/boot_time.svg
+ We also timed the full boot until the login prompt is available, we had a
decrease from ~1 second.
+
+ * The storage test was performed with the fio tool, using a virtio-blk empty
disk. The following arguments were used:
+ fio --filename /dev/vdc --rw=rw --runtime 600 --loops 100 --ioengine libaio
--numjobs 2 --group_reporting
+
+ On average we had a ~4.5% speedup in both reads and writes, the
+ following chart represents the data:
+ https://kernel.ubuntu.com/~gpiccoli/MSI/fio_storage.svg
+
+ * From the network perspective, we've used iPerf with the following
+ arguments: iperf -c <server> -t 300 (server was the host machine). On
+ average, the performance improvement was ~8%, as per the following
+ chart: https://kernel.ubuntu.com/~gpiccoli/MSI/iperf_network.svg
+
+ [Where problems could occur]
+ * Given that the main linux package (generic) and basically all other
derivatives already enable this option, and given that MSIs are the standard
with regards to interrupts from drivers point-of-view, it's safe to say the
risks are minimal, likely smaller than not enabling MSIs (since the hot path is
usually more tested/exercised).
+
+ * That said, problems could occur if we have bugs in MSI-related code in
+ drivers or in PCI MSI core code, then those potential problems that
+ would already affect all other derivatives begin to affect linux-kvm
+ with this change.
** Changed in: linux-kvm (Ubuntu Xenial)
Status: In Progress => Invalid
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https://bugs.launchpad.net/bugs/1914283
Title:
Enable CONFIG_PCI_MSI in the linux-kvm derivative
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