** Changed in: linux-kvm (Ubuntu Groovy)
Status: In Progress => Fix Committed
** Changed in: linux-kvm (Ubuntu Focal)
Status: In Progress => Fix Committed
** Changed in: linux-kvm (Ubuntu Bionic)
Status: In Progress => Fix Committed
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https://bugs.launchpad.net/bugs/1914283
Title:
Enable CONFIG_PCI_MSI in the linux-kvm derivative
Status in linux-kvm package in Ubuntu:
In Progress
Status in linux-kvm source package in Xenial:
Invalid
Status in linux-kvm source package in Bionic:
Fix Committed
Status in linux-kvm source package in Focal:
Fix Committed
Status in linux-kvm source package in Groovy:
Fix Committed
Status in linux-kvm source package in Hirsute:
In Progress
Bug description:
[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.
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