Anyone?

I'm running on a Dell 6650 with 3 physical CPUs (6 cores) with Ubuntu 7.04 on the physical hardware (no skew), vmware server 1.0.3(?) and seeing time skew of about 1.5x on guests running FreeBSD 6.2 and other Linux variants

Help!

Message was edited by: Knobee to add version information

Andrew

General background:

The issue with virtualization and (mostly) Linux guests is, that Linux (and other OS's, too) are using the PIT (Programmable Interrupt Timer) to count the timer ticks. Linux requests a timer interrupt in frequencies of 100 - 1000 Hz per OS instance + per (virtual) processor (n+1). One bad thing about that is, that this will consume ressources, even if the system itself is idle.

The other bad thing is that only the guest which actually owns the processor receives the interrupt, so many interrupts are lost (Lost Timer Tick). That's why the time of a virtualized machine would always run too slow, if there is no compensation.
Windows uses the PIT, too, and other different mechanisms/time sources which can even change if you install software, for example Apple's Quick Time.

As loosing these ticks happens on hardware, as well, there is a Lost Timer Tick correction algorithm within Linux to compensate this. This works well enough on pure hardware.
But there's a second compensation mechanism if you run it onVMware: VMware tracks the interrupts lost and sends them in a timely compressed manner to the guest later (in fact not all are send, some lost ticks needs to be deleted by the Vmware time management). That's the reason why the time doesn't run consistent and seems to "jump" sometimes.

So there are two mechanisms to keep the time accurate and that could lead to a clock running too fast (overcompensation). There is no way to slow it down with the VMware tools, yet, nor do they set the time back. Slowing down the clock is planned for a future release of the tools.

The suboptimal usage of the PIT has changed from Kernel 2.6.18 for 32 Bit / 2.6.21 64 Bit. In newer Kernels Linux uses a different lock source (TSC, Time Stamp Counter) where it can read the time directly (has not to count the ticks to generate the time) and without using an interrupts. Another advantage is that his time device can be read faster that the PIT.

AWo