Kernel Mode Linux

To measure the degree of performance improvement, I conducted two experiments. Both experiments compared performance of the original Linux kernel and KML. I used the sysenter/sysexit mechanism for performance measurement of the original Linux kernel, instead of the int 0x80 instruction. The experimental environment is shown in Table 1.

Table 1. Experimental Environment

In the first experiment, I measured the latency of the getpid and gettimeofday system calls. In the measurement, the system calls were invoked directly by user programs, without libc. The latency was measured with the rdtsc instruction. The result is shown in Table 2.

Table 2. Latency of System Calls (Unit: CPU Cycles)

The result shows that getpid was 36 times faster in KML than in the original Linux kernel, and gettimeofday was twice as fast in KML as it was in the original Linux kernel.

The second experiment is a file I/O benchmark using the Iozone filesystem benchmark. I measured the throughput of four types of file I/O: write, rewrite, read and reread. The measurements were performed on various file sizes from 16KB to 256KB, and the buffer size was fixed at 8KB. The underlying filesystem was ext3. In each measurement, I executed the Iozone benchmark 30 times and chose the best throughput.

The throughput of reread is shown in Table 3. Due to space limitations, the detailed results for write, rewrite and read are omitted.

Table 3. Throughput of reread: Buffer Size=8KB

The result shows that the throughput of reread in KML was improved by 6.8-21%. In addition, write was improved by 0.6-3.2%, rewrite was improved by 3.3-5.3% and read was improved by 3.1-15%.

These experimental results indicate that KML can improve the performance of applications that invoke system calls often, such as those that read or write many small files. For example, web servers and databases can be executed efficiently in KML.

I've performed a benchmark for the Apache HTTP server on KML. It didn't show performance improvement, because I have only a 100-base Ethernet LAN, which became the main bottleneck. If I perform the benchmark on a faster network (say, 1000-base Ethernet or faster), I predict it will show performance improvement.

In the preceeding experiments, it is worth noting that KML eliminated only the overhead of system calls. With some modification to the application, KML will be able to do more for performance improvement. For example, kernel-mode user processes can access I/O buffers directly in the kernel to improve I/O performance.

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