Re: RE: Live migration fails due to c/s 20627

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 12/16/2009 08:23 AM, Dan Magenheimer wrote:
> As Jeremy has pointed out, this cpu/node information is exactly
> the same information that can be obtained by a system call.
> So the only reason that rdtscp is better than using the
> system call would be for performance.
>    

No, not a system call.  The vgetcpu vsyscall will return the info with 
no syscalls, regardless of whether rdtscp is available.  It encodes the 
data in the segment limit of a special segment, and it can be read back 
with the "lsl" instruction.

> Rdtscp is faster than a system call in many situations, but
> now is often emulated in Xen (even on processors that do support
> the hardware instruction*), so cannot be assumed to be much
> faster than a system call.  And the difference in performance
> is only measurable if an app is executing rdtscp many thousands
> of times every second.
>    

"lsl" is probably at least as fast as rdtscp when executed natively, and 
definitely if rdtscp is emulated.

> Suppose a guest believes it has eight cores on a single
> processor/node.
[...]
> Suppose a guest believes it has a total of four cores,
> two cores on each of two nodes.

The pvops kernel never attempts to determine the underlying machine 
topology; it always assumes a single NUMA node.

     J