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Re: Summary of the Arm ports BoF at DC17

On Sun, Sep 17, 2017 at 12:59:04PM -0700, Vagrant Cascadian wrote:
> On 2017-09-15, Ben Hutchings wrote:
> > On Thu, 2017-09-14 at 03:40 +0100, Steve McIntyre wrote:
> > [...]
> >> There is optional kernel support to trap the exceptions here
> >> and emulate the instructions, but it's really not recommended for
> >> serious use (e.g. on a build machine!).
> > [...]
> >
> > Why is it not recommended?  Terrible performance, or known bugs?
> On arm64 kernels building armhf packages for the reproducible builds
> builders,

Steve was mostly talking about armel, I thought?

> I'm seeing hundreds or even thousands of kernel log warnings
> per second when building anything that makes use of
> CP15_BARRIER_EMULATION (e.g. anything using ghc):
>   [85740.553537] cp15barrier_handler: 115316 callbacks suppressed
>   [85740.559358] "ghc-pkg" (29572) uses deprecated CP15 Barrier instruction at 0xf5beaa7c
>   [85740.567344] "ghc-pkg" (29572) uses deprecated CP15 Barrier instruction at 0xf5beaa9c
> That *might* be sufficient to actually impact performance; it certainly
> makes it hard to read other kernel log messages on those machines...

We've been building armhf packages in arm64 VMs for some time in Ubuntu
and it generally seems to work fine; I haven't looked *very* closely but
I haven't noticed any particularly significant performance difference
between armhf and arm64 builds in that environment, including builds
that use GHC.  (I would have to go to some effort to look at kernel
logs, so I haven't.)

We're using HP ProLiant m400 cartridges running as OpenStack compute
nodes, and I gather that hardware supports a few more ARMv7 instructions
than some ARMv8 hardware does.  We also run with a small kernel tweak
[1] to set "uname -m" output to "armv7l" when running under linux32(1),
which makes some dodgy configure scripts behave themselves a bit better.
But apart from that, it's been a pretty good experience on armhf and
definitely much faster than what we had before.

[1] https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/xenial/commit/kernel/sys.c?id=fff84367fa2a19f20c2b9094b47bc3db51fed183

I can't really speak for how it would be on armel, but I rather suspect
that decent ARMv8 hardware would be sufficiently much faster than
ARMv7-or-earlier kit to more than cancel out performance losses from
instruction emulation.

Colin Watson                                       [cjwatson@xxxxxxxxxx]