Use multithreaded "xz" when creating tarballs

[lbrpdx]

Changing a little bit the options for "tar" saves me precious minutes on a build.
On my 2-core/4-thread CPU, I typically save about 20 minutes on a x86_64 build.

When I time the creation of boot.tar.xz:

• tar -cJf (where xz uses only 1 thread):
  real 32m48.042s
  user 32m44.317s
  sys 0m22.580s
• tar -I 'xz -T0' -cf (as many threads as available cores):
  real 13m53.472s
  user 13m10.625s
  sys 0m19.628s

With CPUs with more cores, the win might even be well higher.

[tcamargo]

Fine by me. What about also changing squash fs compression to zstd? Decompression is really fast.

[lbrpdx]

Where is that done, @tcamargo?

[tcamargo]

Where is that done, @tcamargo?

Add BR2_TARGET_ROOTFS_SQUASHFS4_ZSTD=y to your defconfig.

[lbrpdx]

Couldn't make before/after measurement, but Zstd is faster for sure. There is an interesting paragraph about the benefits of Zstd for SquashFS on https://engineering.fb.com/core-data/zstandard/

[tcamargo]

I love when someone remove items. from my todo list. ;)

[lbrpdx]

Unfortunately, doesn't boot any more with Zstd

[mistepien]

I think this is a problem with kernel. squashfs in normal kernel is not compiled with zstd.