Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Coming up in our forums was a testing request to compare the performance of Linux between using 32-bit, 32-bit PAE, and 64-bit kernels. This is coming after Linus Torvalds has spoke of 25% performance differences between kernels using CONFIG_HIGHMEM4G and those without this option that allows 32-bit builds to address up to 4GB of physical RAM on a system. We decided to compare the performance of the 32-bit, 32-bit PAE, and 64-bit kernels on a modern desktop system and here are the results.
For this comparison we used Ubuntu 9.10 on a Lenovo ThinkPad T61 notebook running an Intel Core 2 Duo T9300 processor, 4GB of system memory, a 100GB Hitachi HTS7220 SATA HDD, and a NVIDIA Quadro NVS 140M. We were using the Ubuntu-supplied kernels that are based off the Linux 2.6.31 kernel in Ubuntu Karmic. Other packages that were maintained included GNOME 2.28.1, X Server 1.6.4, NVIDIA 195.22 display driver, GCC 4.4.1, and we were using the default EXT4 file-system with all other defaults. With Ubuntu to properly address 4GB or greater of system memory you need to use a PAE kernel as the Physical Address Extension support through the kernel's high-mem configuration options are not enabled in the default 32-bit kernels. CONFIG_HIGHMEM4G is enabled in the default Ubuntu kernel, but the Ubuntu PAE kernel uses CONFIG_HIGHMEM64G (and other build options) for handling up to 64GB of system memory. Of course, with 64-bit addressing there is not this greater than 4GB RAM limitation. Though even with a 32-bit non-PAE kernel the system will only report 3GB of system memory by default due to 1GB of that being reserved for kernel virtual addresses while the 3GB is available to user-space addresses.