On 11/12/20 22:04, Thomas Gleixner wrote:
Its 100ms off with migration, and can be reduced further (customers
complained about 5 seconds but seem happy with 0.1ms).
What is 100ms? Guaranteed maximum migration time?
I suppose it's the length between the time from KVM_GET_CLOCK and
KVM_GET_MSR(IA32_TSC) to KVM_SET_CLOCK and KVM_SET_MSR(IA32_TSC). But
the VM is paused for much longer, the sequence for the non-live part of
the migration (aka brownout) is as follows:
pause
finish sending RAM receive RAM ~1 sec
send paused-VM state finish receiving RAM \
receive paused-VM state ) 0.1 sec
restart /
The nanosecond and TSC times are sent as part of the paused-VM state at
the very end of the live migration process.
So it's still true that the time advances during live migration
brownout; 0.1 seconds is just the final part of the live migration
process. But for _live_ migration there is no need to design things
according to "people are happy if their clock is off by 0.1 seconds
only". Again, save-to-disk, reverse debugging and the like are a
different story, which is why KVM should delegate policy to userspace
(while documenting how to do it right).
Paolo
CLOCK_REALTIME and CLOCK_TAI are off by the time the VM is paused and
this state persists up to the point where NTP corrects it with a time
jump.
So if migration takes 5 seconds then CLOCK_REALTIME is not off by 100ms
it's off by 5 seconds.
CLOCK_MONOTONIC/BOOTTIME might be off by 100ms between pause and resume.
