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.
The only differences in the kernel configuration between Ubuntu's PAE and non-PAE 32-bit kernels are enabling the CONFIG_X86_CMPXCHG64, CONFIG_HIGHMEM64G instead of CONFIG_HIGHMEM4G, CONFIG_X86_PAE, CONFIG_ARCH_PHYS_ADDR_T_64BIT, CONFIG_PHYS_ADDR_T_64BIT, CONFIG_I2O_EXT_ADAPTEC_DMA64, and disabling CONFIG_ASYNC_TX_DMA. The rest of the kernel configuration is the same. The Linux kernel also requires that the CPU itself supports PAE, but these days that is practically all Intel and AMD processors.
Among the tests we ran on the three Linux 2.6.31 kernels with the Phoronix Test Suite were OpenArena, Apache, PostgreSQL, Bullet, C-Ray, Gcrypt, GnuPG, GraphicsMagick, timed MAFFT alignment, John The Ripper, OpenSSL, x264, and PostMark.
With the ioquake3-powered OpenArena game there were virtually no performance differences between the 32-bit, 32-bit PAE, and 64-bit kernels. We had ran other OpenGL-powered tests too through the Phoronix Test Suite and found no significant differences, so we are just sharing one set of numbers in this article to avoid repetition.
While the different kernels had not affected the gaming performance with our Core 2 Duo laptop running with 4GB of system memory, the Apache performance was significantly affected. The stock Ubuntu 32-bit kernel had managed to 473 requests per second while the PAE kernel dropped just slightly with its 467 request average, but meanwhile the 64-bit support was many times faster with its 7,989 requests per second count.