IPI Benchmark Data?

[Alex Merritt]

Matt,

That's awesome, thanks for sharing. For the Broadwell system, it seems to
take 2-3 microseconds in C-state 1. By any chance do you have numbers for
C-state 0? I am curious what an IPI costs for a non-idle cpu, too. These
numbers are interesting, though, I didn't realize wake-ups cost so much (or
to think about whether to send an IPI to an idle core in the first place).

If cpu #16 is the neighbor SMT thread for cpu #0, wouldn't IPI latencies be
minimal? I assumed both SMT threads couldn't be in different C-states since
they share resources. Unless I am reading the data incorrectly...

-Alex



On Wed, Aug 3, 2016 at 12:29 PM, Matthew Dillon <dillon at backplane.com>
wrote:

> I have some saved output from ./ipitest script runs that I've run on
> various cpus:
>
> http://apollo.backplane.com/DFlyMisc/ipitest01.txt
>
> http://apollo.backplane.com/DFlyMisc/ipitest02.txt
>
> http://apollo.backplane.com/DFlyMisc/ipitest03.txt
>
> http://apollo.backplane.com/DFlyMisc/ipitest04.txt
>
> http://apollo.backplane.com/DFlyMisc/ipitest05.txt
>
> The numbers are in nanoseconds, so e.g. '2900' would be 2.9uS. '29000'
> would be 29uS.  The originating cpu is #0.  As an example, taking
> ipitest05.txt, due to the topological layout cpu #16 is the co-thread for
>  cpu #0 and one can see it in the numbers (since cpu #0 is initiating the
> test, it's generally not running in a low-power mode regardless of the C
> state we use).
>
> The results generally follow expectations.  A cpu sitting in a deep C
> state can take over 30uS to respond.  Minimum IPI latency is usually in the
> 1-2uS range.
>
> ​-Matt
>
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