On 9/26/13 5:50 PM, Roger Pau Monné wrote:
On 26/09/13 03:48, Julian Elischer wrote:
On 9/25/13 11:04 PM, Roger Pau Monné wrote:
On 25/09/13 16:56, Julian Elischer wrote:
If CPUs are mapped around, how are IPIs handled? I assume they must be
emulated?

I've noticed that under Xen (on both Amazon EC2 and a Redhat server)
whenever you schedule a thread it always sits on the run queue for 20
uSecs before it starts running. It looks to me like it's the IPI taking
a long time to be emulated.
This has been improved on the FreeBSD Xen PVHVM port by using PV IPIs
instead of the emulated ones, see r255331. It should be faster than the
previous emulated implementation.

I missed that.. thanks!
Do you (or anyone else) know if this can be used on Amazon EC2?
And do you need a specific version/configuration of Xen to be able to
use it?

I ran a new GENERIC kernel on an Amazon Xen 4.2 system and it performed a bit better. delays in the running of threads reduced from a constant 20uSecs to a variable amount between 1.5uSecs and 9uSecs.
It's still very slow when compared with real hardware.
(timing viewed using Schedgraph)

Is there a way to see whether PV IPIs are active?
(it looks it from the results but that could be so many other things too).

Julian
The PV IPIs require Xen version 4.0 or greater, and the PV timer
requires 4.0.1 or greater if I'm not mistaken.

If you are lucky to get an Amazon instance that uses this Xen version
(or any superior one) they will be activated by default, if not FreeBSD
will switch to the old event delivery method and PV IPIs and PV timer
will be disabled in favour of the emulated ones.

I guess it's just a matter of time before Amazon switches all their
servers to Xen 4.x.

Roger.




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