Episode Details

Jordan Hendricks joined Bryan and Adam to talk about her work virtualizing time--particularly challenging when migrating virtual machines from one physical machine to another!

We've been hosting a live show weekly on Mondays at 5p for about an hour, and recording them all; here is the recording from June 12th, 2023.

In addition to Bryan Cantrill and Adam Leventhal, we were joined by Oxide colleague Jordan Hendricks.

The (lightly edited) live chat from the show:

• DanCrossNYC: The TSC ticks at a fixed rate now days, regardless of voltage scaling on the CPU.
• jbk: just x86 doesn't provide a consistent want to determine what the rate is
• jbk: (I guess some chips will tell you via CPUID, but I've yet to actually encounter such chips)
• jbk: some hypervisors will tell you via an MSR
• zorg24: Looks the Linux kernel docs have some documentation on the x86 TSC and PIT https://www.kernel.org/doc/html/next/virt/kvm/x86/timekeeping.html
• DanCrossNYC: CPUID or an MSR, but yeah, most systems sample over a fixed interval (determined by another time source) to figure it out.
• jbk: no, versus some other present component that allows you to measure the frequency
• DanCrossNYC: No, the PIT or HPET or something.
• jbk: https://src.illumos.org/source/xref/illumos-gate/usr/src/uts/i86pc/os/tscc_pit.c?r=236cb9a8
• jbk: is how it uses the PIT
• jbk: (the HPET code needs to improve it's accuracy, so it's only used when the PIT isn't there at the moment)
• jbk: some Intel NUCs have no PIT
• jbk: so HPET is the only option
• bcantrill: https://github.com/illumos/illumos-gate/commit/717646f7112314de3f464bc0b75f034f009c861e
• DanCrossNYC: Two big ones: system maintenance without disturbing guest workloads, and also load balancing across a rack.
• "Sevan: ah, thanks.
• https://github.com/illumos/illumos-gate/blob/717646f7112314de3f464bc0b75f034f009c861e/usr/src/test/bhyve-tests/tests/common/common.c#L166"
• bcantrill: https://github.com/oxidecomputer/tsc-simulator/tree/master
• DanCrossNYC: The guest may well be running NTP itself.
• iangrunert: I assume you could also check that NTP is alive / has synced recently before doing a migration right?
• aka_pugs: Do people use IEEE 1588/PTP in datacenters? Maybe finance wackos?
• zorg24: also it might be tricky to check if NTP synced recently if it is happening in usermode
• iangrunert: Might've missed this - is it just the hypervisor that has to run NTP recently or the VM as well?
• saone: I believe it was just the hypervisor
• DanCrossNYC: The host.
• DanCrossNYC: A guest may or may not; that's up to the guest.
• jbk: but IIUC, if the guest IS running NTP, then the host definitely needs it to a