Use smaller initramfs compression by default and in most devices

!334 (merged) will allow using a different initramfs compression method. Let's use a better compression type (once it is merged).

• Figure out what a good default is (a value that is supported by all kernels we have, is that true for gbzip2)?
• Enforce that to be enabled in the kernel (which option is it exactly?) with pmbootstrap kconfig check
• Set this value in all existing devices
• Set it in new devices by default (adjust pmbootstrap aportgen linux code)
• Consider removing unused compression algorithms from postmarketos-mkinitfs again (the ones that were added in !334 (merged))

added help wanted type::feature labels

mentioned in merge request !334 (merged)

By Oliver Smith on 2019-04-28T22:40:48

The configs that provide ramdisk decompressors are CONFIG_RD_*. These configs depend on CONFIG_BLK_DEV_INITRD.

That config has been a default in Android for a very long time:
https://android.googlesource.com/kernel/common/+/refs/heads/deprecated/android-3.0/android/configs/android-base.cfg#12
https://android.googlesource.com/kernel/common/+/refs/heads/deprecated/android-3.4/android/configs/android-base.cfg#14
https://android.googlesource.com/kernel/common/+/refs/heads/deprecated/android-3.10/android/configs/android-base.cfg#17
https://android.googlesource.com/kernel/configs/+/refs/heads/pie-release/o/android-4.9/android-base.cfg#15

I think CONFIG_BLK_DEV_INITRD should be enforced.

Based on usr/Kconfig, gzip provides a balance between compression ratio and speed. lzma and xz provide the highest compression ratio. gzip and lzma have been around since at least 2.6.30-rc1.

https://github.com/torvalds/linux/commit/30d65dbfe3add7f010a075991dc0bfeaebb7d9e1#diff-628d73975944fcb2127963d969dc0790

gzip seems like a sensible default to me. Unless we want to enforce lzma which has a better compression ratio. Maybe document in the wiki to try lzma or xz if a device has a very small boot partition.

By ryang on 2019-04-29T14:59:08

I tested the compression ratio of samsung-p4wifi ramdisk using the "real" binaries (not busybox).

compression	size (bytes)
bz2 -1	1,213,033
bz2 -9	1,215,152
gzip -1	1,216,950
gzip -9	1,208,037
lz4 -1	1,429,705
lz4 -9	1,340,368
lzma -1	1,084,331
lzma -9	1,044,231
xz -1	1,084,540
xz -9	1,044,428

(EDIT by ollieparanoid: added commas to the numbers)

By ryang on 2019-04-30T17:24:58

changed the description

By Oliver Smith on 2019-04-30T17:11:04

mentioned in merge request pmbootstrap!1780 (merged)

By Oliver Smith on 2019-04-30T17:39:02

Thanks for the research!

I think CONFIG_BLK_DEV_INITRD should be enforced.

https://gitlab.com/postmarketOS/pmbootstrap/merge_requests/1780

Maybe document in the wiki to try lzma or xz if a device has a very small boot partition.

At least the deviceinfo reference is pointing to this thread now, so people can get to your measurements.

I tested the compression ratio of samsung-p4wifi ramdisk using the "real" binaries (not busybox).

I'm surprised that we can't get that much smaller than gzip -1. So it seems to me like we don't really need to optimize this - except for devices which have such a tiny partition that it really matters (like the htc-ace). And since we have the deviceinfo variable now, we can do it in a device specific way already. So I'm closing this for now, but we could reopen it if someone wants to discuss this more.

By Oliver Smith on 2019-04-30T17:44:35

closed

By Oliver Smith on 2019-04-30T17:44:36