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Re: [Xen-devel] RE: Live migration fails due to c/s 20627

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: Re: [Xen-devel] RE: Live migration fails due to c/s 20627
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Wed, 16 Dec 2009 09:38:25 -0800
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, kurt.hackel@xxxxxxxxxx, "Dugger, Donald D" <donald.d.dugger@xxxxxxxxx>, "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>
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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
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.


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