Comparing Linux 2.4 and Linux 2.6 Kernels:

Many customers ask for support of Linux Kernel version 2.6 for their embedded projects. This is easy enough to understand: after all, 2.6 is the latest and greatest version of the Linux kernel, and who wants to base a new project on old stuff?

Here is a comparison of 2.4 and 2.6 on real embedded hardware:

Build and Test Environment, Kernel Versions and Test Hardware:

The ELDK 3.1 toolchain was used to build the Linux kernels and to provide the necessary test environment (NFS root filesystem, ramdisk images).

The following versions of the Linux Kernel were used in the tests:

• Kernel Version 2.4.25 from CVS server at www.denx.de, snapshot of 2005-04-23-00:00
• Kernel Version 2.6.11.7 from kernel.org plus 8xx v2.6 TLB patch by Marcelo Tosatti (4 Apr 2005)

The following hardware was used for the tests:

• Sandpoint 8240 (MPC8240 at 132 MHz CPU clock, 64 MB RAM)
• TQM860L (MPC860-T at 50 MHz CPU clock, 16 MB RAM)

Building the Kernels:

2.4.25:

• Sandpoint Board:

  -> export CROSS_COMPILE=ppc_82xx-
  -> make sandpoint_config
  -> make oldconfig
  -> time make -j8 dep uImage
  ...
  Image Name:   Linux-2.4.25
  Created:      Sat Apr 23 16:11:01 2005
  Image Type:   PowerPC Linux Kernel Image (gzip compressed)
  Data Size:    909254 Bytes = 887.94 kB = 0.87 MB
  ...
  real    1m17.691s
  user    4m28.245s
  sys     0m28.346s
  -> size vmlinux
     text    data     bss     dec     hex filename
  1838116  125624  172708 2136448  209980 vmlinux
  

• config-2.4.25-Sandpoint: Linux 2.4.25 config file for Sandpoint board
• uImage-2.4.25-Sandpoint: Linux 2.4.25 U-Boot image for Sandpoint board

• TQM860L Board:

  -> export CROSS_COMPILE=ppc_8xx-
  -> cp ../config-2.4.25-TQM860L .config
  -> make oldconfig
  -> time make -j8 dep uImage
  ...
  Image Name:   Linux-2.4.25
  Created:      Sat Apr 23 17:04:44 2005
  Image Type:   PowerPC Linux Kernel Image (gzip compressed)
  Data Size:    507688 Bytes = 495.79 kB = 0.48 MB
  ...
  real    0m50.348s
  user    2m55.328s
  sys     0m19.589s
  -> size vmlinux
     text    data     bss     dec     hex filename
  1025524   50008  142596 1218128  129650 vmlinux
  

• config-2.4.25-TQM860L: Linux 2.4.25 config file for TQM860L board
• uImage-2.4.25-TQM860L: Linux 2.4.25 U-Boot image for TQM860L board

2.6.11.7:

• Sandpoint Board:

  -> export CROSS_COMPILE=ppc_82xx-
  -> make ARCH=ppc CROSS_COMPILE=ppc_82xx- sandpoint_defconfig
  -> time make -j8 ARCH=ppc CROSS_COMPILE=ppc_82xx- uImage
  ...
  Image Name:   Linux-2.6.11.7
  Created:      Sat Apr 23 16:42:11 2005
  Image Type:   PowerPC Linux Kernel Image (gzip compressed)
  Data Size:    1150438 Bytes = 1123.47 kB = 1.10 MB
  ...
  real    1m41.571s
  user    5m48.509s
  sys     0m39.444s
  -> size vmlinux 
     text    data     bss     dec     hex filename
  2253870  226316   93720 2573906  274652 vmlinux
  

• config-2.6.11.7-Sandpoint: Linux 2.6.11.7 config file for Sandpoint board
• uImage-2.6.11.7-Sandpoint: Linux 2.6.11.7 U-Boot image for Sandpoint board

• TQM860L Board:

  -> export CROSS_COMPILE=ppc_8xx-
  -> cp ../config-2.6.11.7-TQM860L .config
  -> make ARCH=ppc CROSS_COMPILE=ppc_8xx- oldconfig
  -> time make -j8 ARCH=ppc CROSS_COMPILE=ppc_8xx- uImage
  ...
  Image Name:   Linux-2.6.11.7
  Created:      Sat Apr 23 17:14:19 2005
  Image Type:   PowerPC Linux Kernel Image (gzip compressed)
  Data Size:    723718 Bytes = 706.76 kB = 0.69 MB
  ...
  real    1m12.198s
  user    4m11.517s
  sys     0m29.351s
  -> size vmlinux
     text    data     bss     dec     hex filename
  1497745   79248   64248 1641241  190b19 vmlinux
  

• config-2.6.11.7-TQM860L: Linux 2.6.11.7 config file for TQM860L board
• uImage-2.6.11.7-TQM860L: Linux 2.6.11.7 U-Boot image for TQM860L board

Results:

Sandpoint	2.4.25	2.6.11.7	Delta
real	77.7s	101.6s	+ 31 %
user	268.2s	348.6s	+ 30 %
sys	28.3s	39.4s	+ 39 %
Compressed Kernel	887.9kB	1123.5kB	+ 27 %
Uncompressed kernel	2086.4kB	2513.6kB	+ 20 %

TQM860L	2.4.25	2.6.11.7	Delta
user	175.3s	251.5s	+ 43 %
Uncompressed kernel	1189.6kB	1602.8kB	+ 35 %
sys	19.6s	29.4s	+ 50 %
real	50.3s	72.2s	+ 43 %
Compressed Kernel	495.8kB	706.8kB	+ 43 %

Booting Linux:

The kernel images were booted in the same environment with root filesystem over NFS in an idle network. The timings were recorded in the following log files:

• BOOTLOG-2.4.25-TQM860L: Timed bootlog for Linux 2.4.25 on TQM860L board
• BOOTLOG-2.4.25-Sandpoint: Timed bootlog for Linux 2.4.25 on Sandpoint board
• BOOTLOG-2.6.11.7-TQM860L: Timed bootlog for Linux 2.6.11.7 on TQM860L board
• BOOTLOG-2.6.11.7-Sandpoint: Timed bootlog for Linux 2.6.11.7 on Sandpoint board

Results:

Time from begin of loading the kernel image into RAM ...	2.4.25	2.6.11.7	Delta
... until "Linux version" message
TQM860L	1.9s	2.0s	+ 6 %
Sandpoint	13.0s	15.0s	+ 15 %
... until "Freeing unused kernel memory" message (= enter user space)
TQM860L	3.7s	3.8s	+ 1 %
Sandpoint	23.0s	22.0s	- 4 %
... until "login:" message (= full multi-user mode)
TQM860L	49.7s	58.0s	+ 17 %
Sandpoint	56.2s	59.6s	+ 6 %

The slight advantage for the 2.6 when booting is at least partially due to the fact that the configurations were slightly different: in 2.4, the SCSI driver was enabled, which was not the case in 2.6

Running with ramdisk based root file system:

The systems were booted using the ramdisk images provided by the ELDK. The main purpose was to find out how much memory remained free for application code.

Running the lmbench benchmark:

To get some more extensive performance data the lmbench benchmark (version 3.0-a4) was run on the systems). Here the results:

Management Summary:

Using the 2.6 kernel on embedded systems implicates the following disadvantages:

• Slow to build: 2.6 takes 30...40% longer to compile
• Big memory footprint in flash: the 2.6 compressed kernel image is 30...40% bigger
• Big memory footprint in RAM: Run with rootfs in ramdisk and run "free" command!!!
• Slow to boot: 2.6 takes 5...15% longer to boot into multi-user mode
• Slow to run

-- WolfgangDenk - 23 Apr 2005