Yocto part II – baseline image size

In the first installment of this series, we established a base line image and had a look at the initial boot performance. This time, we will establish a baseline for the image size. In an embedded system, these two factors often go hand in hand, as reading data from FLASH often consumes a considerable amount of time during boot.

It is fairly easy to see how large the resulting root file system and kernel are, but we need more detail. To enable some statistics for the sizes of the various packages built, we first need to enable build history and rebuild our baseline image. Build history is enabled by adding the following lines to the local.conf file.

INHERIT += "buildhistory"
BUILDHISTORY_COMMIT = "1"


This produces a set of files with statistics in buildhistory/images/qemux86/eglibc/core-image-minimal. The files-in-image.txt file, a detailed list of all the files of the system, along with their sizes, is listed. In the installed-package-sizes.txt, we get a list of responsible packages. The list for a baseline image can be seen below.

8641 KiB systemd
4487 KiB udev-hwdb
3042 KiB libc6
1290 KiB libx11-6
1221 KiB e2fsprogs-e2fsck
1153 KiB shadow
798  KiB dbus-1
547  KiB kmod
532  KiB busybox
398  KiB udev
350  KiB libkmod2
299  KiB libdbus-1-3
297  KiB libext2fs2
261  KiB libmount1
249  KiB udev-utils
241  KiB libblkid1
233  KiB systemd-analyze
161  KiB liblzma5
159  KiB libexpat1
133  KiB libxcb1
110  KiB v86d
94   KiB util-linux-fsck
86   KiB libz1
83   KiB libgcc1
36   KiB systemd-binfmt
32   KiB kernel-module-uvesafb
31   KiB util-linux-mount
31   KiB libwrap0
30   KiB libacl1
29   KiB util-linux-agetty
27   KiB libe2p2
23   KiB netbase
20   KiB kernel-3.14.0-yocto-standard
19   KiB libxdmcp6
16   KiB libattr1
15   KiB libcap2
14   KiB libuuid1
11   KiB kernel-module-binfmt-misc
10   KiB libcom-err2
9    KiB libxau6
5    KiB update-rc.d
4    KiB update-alternatives-opkg
4    KiB base-files
3    KiB busybox-udhcpc
2    KiB shadow-securetty
2    KiB run-postinsts
1    KiB systemd-serialgetty
1    KiB busybox-syslog
0    KiB systemd-compat-units
0    KiB packagegroup-core-boot
0    KiB base-passwd


In addition to this, the bz kernel image is 6.2MiB.

My approach to this system will be that of a gadget maker, so when trying to optimize the size of the system, I will try to use as much as possible of what I know of the target hardware to minimize the size. This ought to make it possible to reduce the kernel, the udev-hwdb and more. In addition to this, Lennart says that systemd is tweakable when it comes to size. I’m looking forward to trying this.

That is all for this installment. Next time we will have a look at setting up our own meta layer to start playing with, then we will try to create a truly minimalist and fast booting image.