[Buildroot] Build time increase in 2018.02

[Trent Piepho <tpiepho@impinj.com>]

On Sat, 2018-03-10 at 12:32 +0100, Peter Korsgaard wrote:
> > > > > >  > We've seen a big increase in build time with the latest buildroot.  On
>  > a vpshere instance, times have gone up from 45 minutes to 180+ minutes.
> 
> Wow! What version did you use before upgrading to 2018.02 (the 45min)?

2017.11.1.  I see one change that went in between that and 2018.02 is,
"core/pkg-generic: store file->package list for staging and host too"

If I breakdown step_pkg_size by tree:
step_pkg_size-stage      143.50
step_pkg_size-target     267.14
step_pkg_size-host       419.21

The other targets, extract, build, etc. are <1 second.  So adding
package size stats for staging and host is responsible for tripling the
time this step takes.

Looking@how the file accounting is done, it will md5sum the tree
with complexity O(n^2) on the size of the tree.  So it is not
surprising that it is very slow.  It also explains why re-installing a
host package after the build is done slow, since it must md5sum the
entire host tree twice.  At least when building it takes about half as
long since the earlier packages to install have a smaller tree to sum.


>  > step_pkg_size        831.17
>  > configure            441.77
>  > build                149.56
>  > extract               40.28
>  > host-install          31.96
>  > all other             27.78
>  > check_host_rpath      20.86
>  > check_bin_arch        20.77
>  > target-install        19.84
>  > stage-install         13.25
> 
>  > Finding the package sizes takes more time than everything else put
>  > together.  And indeed, if I remove that step, check_bin_arch, and fake
>  > out check-uniq-files, the build drops from 26.5 minutes to 12 minutes!
> 
> Argh, that's pretty horrible :/ I have noticed some overhead from the
> various instrumentation steps, but nothing as significant as this.
> 
> Out of interest, what kind of storage are you building this on? SSD or
> spinning disk?

A good NVMe SSD using PCI-Express.

>  > On the vsphere instance, the time is much much worse.  I'm still
>  > waiting for numbers.  It takes hours.  I think the lousy vm filesystem
>  > performance magnifies the problem.
> 
> Yes, I/O performance is absolutely critical for stuff like Buildroot.

Here's the time for running on a VM.  
targetinstall             29.65
stageinstall              31.57
check_bin_arch            34.11
post_image                38.63
check_host_rpath          41.23
hostinstall               55.40
extract                   72.99
other                     73.77
build                    465.93
configure                689.38
step_pkg_size            2872.76

47 minutes to check the package sizes.

While I don't use a VM myself, the people who run the infrastructure
for the CI and nightly builds think they are great.  It's the way
things are now.  Everyone's IT dept uses vsphere or AWS or some other
tech to allow them to create instances that are decoupled from the
physical hardware present (or in the cloud).