Re: [gentoo-embedded] Mixed arch chroot to optimise building in qemu

[Ed W]

On 22/09/2021 20:26, Michael Jones wrote:
>
>
> On Wed, Sep 22, 2021 at 1:20 PM Ed W <lists@××××××××××.com <mailto:lists@××××××××××.com>> wrote:
>
>     Hi all, traffic seems to have dropped off here significantly, but here goes
>
>     I am building a bunch of armv7a images on an AMD Ryzen9 machine (amd64). So to keep things
>     simple I
>     have just been doing the whole thing using qemu up until now, by which I mean I have an arm
>     stage 3
>     somewhere, I chroot into it and then using userspace qemu binaries I just run my whole script to
>     generate the target build from inside that chroot. This works but it's at least a 5x slowdown from
>     native
>
>     To optimise this I have tried
>
>     - turning on the various compiler options for python (claimed to give a 30% improvement) +
>     LTO/PGO.
>     I don't notice any difference in the chroot - presume that the emulation overhead is dominant
>     effect
>
>     - tried compiling qemu with -O3 and LTO (claimed to be supported since 6.0). Doesn't give any
>     noticeable different in performance of emerge
>
>     - Added a static compiled amd64 /bin/bash to the chroot - now this does give a noticeable boost to
>     compile and emerge speeds. (random benchmark went from 26s to 22s)
>
>
>     So motivated by the last item I want to try and see how many native exes I can push into the
>     chroot
>     (since I'm running under usermode qemu! why not!). The obvious one is the compiler
>
>     Now, I have a cross compiler built, but a) that's not static, so I would need to find a way to get
>     native libc into the chroot, and b) I'm not clear how I would call it inside the chroot, could I
>     just move a symlink to the other compiler into the path? How does it find things like
>     libgcc*.so etc?
>
>     Or perhaps this is easier than this? Can I just use some incantation in the same way that the
>     crosscompiler must be working to build myself a straight gcc inside the chroot which is native
>     arch
>     and statically compiled? eg is it enough that assuming I can build gcc static, can I just do this
>     from outside the chroot and overwrite the native:
>
>         ROOT=$PWD emerge -1v --nodeps gcc
>
>
>     It seems to me that this should work at least for the gcc binaries, etc. However, I'm completely
>     ignorant of whether I want things like the linker plugin in native arch or target arch? What about
>     the libgcc*.so files? (They don't actually exist in my cross compiler directories, but they are
>     linked in as dependencies in some binaries in target and exist in the native compiler dir)
>
>     Hacker news had someone do this recently and I believe meego used to do something similar, so
>     really
>     just trying to work out the details for this on gentoo. Any thoughts?
>
>     Thanks
>
>     Ed W
>
>
>
> It's not clear to me if you're building gentoo images, or just building some application.
>
> If you're building gentoo images, you might consider this project https://github.com/GenPi64
> <https://github.com/GenPi64> , we'd love to work with you on the mixed arch situation, since we
> suffer the same problem.


These are whole gentoo images. :-)

So it's nothing special, but something like I drop into the arm chroot, then there is a whole pile
of something like:

    ROOT=/mnt/new_image emerge $stuff

And at the end of all of that you have a shiny image to boot from (on an imx based SOM as it happens). 

Nice thing about this approach is that I need to build the same system for i386, amd64 and 32bit
arm, and basically it means only running the same build script in each individual chroot, so it's
quite nice not needing to fixup stuff for each platform.


There are arm64bit boxes you can rent from AWS and similar, but we see a few build oddities on this
which still need fixing and at least as near as I can see they are still quite a bit slower than
using an intel processor in native mode.


I'm just about to (re) try using distcc, which basically achieves the required end goal, so that I
can measure performance. So something like run up a side by side chroot using crossdev, then fire up
distcc in there and talk to it from your arm chroot. This gives less speedup than you would like
because it needs quite a lot of work on the arm qemu side and serialising stuff, etc. Also linking
etc is still on the arm side.

I think the replacing of the bash binary with a native static binary is giving a decent speedup. I'm
about to try swapping in pypy to see how that behaves.

However, there is no doubt that getting the native cross compiler into the chroot is the solution,
but there are more than a few challenges here, such as how to get it statically compiled and how to
insert some or all of it into the arm chroot.

See here for inspiration and I guess also the meego stuff from history:

    https://news.ycombinator.com/item?id=28376447


Thanks for any tips!

Ed W