Re: Tweak Latency on Intel ATOM

["Max Müller" <mxmr@gmx.net>]

Luis Claudio R. Goncalves schrieb:
> On Tue, Feb 16, 2010 at 07:50:02AM +0100, Max Müller wrote:
> | Clark Williams schrieb:
> | >On Mon, 15 Feb 2010 10:32:54 +0100
> | >Max Müller <mxmr@gmx.net> wrote:
> | >>>I'd say you're doing pretty good keeping under 50us. You might want to
> | >>>try it under a heavier load than the shell script you've been running.
> | >>>If you don't want to fool with rteval, try kicking off a kernel compile
> | >>>in another window like this:
> | >>>
> | >>>$ while true; do make -j4 clean bzImage modules; done
> | >>>
> | >>>and then run cyclictest. A kernel compile with parallel jobs (-j) is a
> | >>>good overall load of computation and I/O.
> | >>>
> | >>I tested now like you told me with irqbalance and cpuspeed
> | >>services disabled. I hope i made the right for disabling
> | >>irqbalance, i used the kernel parameter acpi_no_irqbalance. Is
> | >>this correct? Unfortunately the results were nearly equal as
> | >>before.
> | >
> | >I don't think you're going to get much better results on the Atom. I
> | >have an MSI Nettop box with the dual-core version and I saw about the
> | >same results as you.
> | >
> | >What sort of scheduling deadlines are you trying to meet?
> | >
> | >Clark
> | Shorter it is better it would be :-)
> | I can also live with this results, but i wanted to make sure to get
> | the best out of this hardware.
> | 
> | Are you running both cores on the MSI box (maybe also with
> | hyperthreading enabled) with this results?
>
> Oops, I have overlooked that "more than one core" detail. In this case, you
> have two cyclictest threads (from different executions) clashing at
> priority FIFO:94. That can eventually create the latencies you see.
>
> I would also suggest running the test without Hyperthreading (adjusting the
> number of cyclictest threads) just to see if the latencies are there in
> this case.
>  
> Luis
>
> | In the meantime i thought also if the SMI (system management mode)
> | could have a bad influence. I wrote a little userspace  programm
> | which disables global SMI bit of the ICH7 southbridge. But also no
> | better results.
> | After that i was told (thanks to Luis Claudio!) to check latency
> | with the kernel module hwlat_detector. The results of this module
> | was 0. I interpreted this that there is no SMI that causes the
> | latency on my ATOM system.
> | 
> | Regards,
> | Max
>
>   
I have only the single core ATOM. Clark has the dual core box. 
Hyperthreading must be disabled, else the maximum latency rises from 
about 50µs (Hypertrhreading disabled) to about 100µs (Hypertrhreading 
enabled).
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