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Re: latency test results for armhf vs arm64?

On Sat, Jul 16, 2022 at 2:41 PM gene heskett <gheskett@shentel.net> wrote:
> On 7/16/22 08:10, Arnd Bergmann wrote:
> >
> > - 4.19 is four years old, and both the mainline kernel and the
> >    preempt-rt patches have changed a lot in the meantime. It's
> >    possible that a current preempt-rt has regressed compared to
> >    the version you are running. If so, we can work on fixing the
> >    regression for future kernels, but there won't be much interest
> >    in working on the old kernel
> >
> > - Raspberry Pi OS (and Raspbian before that) has a number of
> >    platform specific kernel patches that are neither in mainline
> >    Linux nor in the Debian kernel packages. It is possible that they
> >    have already identified and fixed a source of latency in their
> >    kernels but not managed to upstream that fix for a number of
> >    reasons.
> Their kernels are uniformly horrible with latency's ranging to above a
> millisecond.
> Linuxcnc simply refuses to run on those kernels.

I think this is simply because raspbian does not ship any preempt-rt
kernel themselves, so clearly their kernel binaries won't be low-latency.

You did not say where  you got the kernel that you are running
successfully, so as far as I could tell this might be a combination
of the raspbian patches and the preempt-rt patches.

> > - The raspbian user space should have very little effect on
> >     latency but it's worth pointing out that you may see different
> >     performance between armv6 (raspbian)
> I wish you would admit that the raspios I am running IS armhf (kernel7l)
> I've no clue where you got the impression it was v6. It is not.

This paragraph was about the user space, not the kernel.

The entire reason for Raspbian's existence is that it runs on armv6
hardware like the Raspberry Pi 1, which Debian armhf does not
run on. Building for armv6 means they can run the same user space
on all hardware generations from v6 to v8, and they advertise this
on their website:
ship at least two separate 32-bit kernels, since an LPAE-enabled
kernel is needed to access PCI and high memory but is incompatible
with Armv6 hardware.


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