Kernel Korner - Kernel Mode Linux for AMD64

When user code runs inside the kernel, system calls become function calls, 50 times faster. How does that affect the performance of a real application, MySQL?
Performance Measurement

To see how much performance improvement is possible, I ran the Wisconsin benchmark for MySQL both on the original Linux kernel and on KML, using sql-bench, which comes with MySQL. The experimental environment is shown in Table 1. In the test on KML, both the MySQL server and the benchmark client were executed as kernel-mode user processes, and the patched GNU C Library was used to eliminate the overhead of system call invocations. In addition, the loop count of the test was increased to 10,000, as the default loop count of 10 was too small to produce meaningful results.

Table 1. Experimental Environment

CPUOpteron 850 (2.4GHz, L2 cache 1MB) x 4
Memory8GB (Registered DDR1-333 SDRAM)
Hard disk146GB (Ultra320 SCSI 73GB x 2, RAID-0, XFS)
OSLinux kernel 2.6.11 (KML_2.6.11_002)
LibcGNU libc 2.3.5 + patch for KML
MySQLMySQL 4.1.11

The result is shown in Table 2. The second column shows the total CPU time consumed by the benchmark. The third and forth columns show the breakdown of the total CPU time. The third column shows the CPU time consumed by the user process, and the forth column shows the CPU time consumed by the kernel.

Table 2. Result of Wisconsin Benchmark (in seconds)

Original Linux753.86611.78142.08

The results show that the total CPU time was improved by about 3%. The user CPU time was improved by about 1%, and the system CPU time was improved by about 14%. The result indicates that KML could improve the performance of database applications slightly by eliminating the overhead of system call invocations.

Conclusion and Future Work

KML is a modified Linux kernel that can execute user processes in kernel mode. By executing in kernel mode, the performance of user programs can be improved by, for example, eliminating the overhead of system call invocations. Besides the performance improvement, KML also can be used to ease inspection and debugging of the kernel and development of kernel modules, because kernel-mode user processes can access the kernel and use large amount of memory and CPU time. I now am considering implementing a helper library to provide kernel-mode user processes with an easy way to access kernel functions and data by exporting them as some kind of shared object.

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Toshiyuki Maeda is a PhD candidate in Computer Science at the University of Tokyo. His favorite comic artists are Osamu Tezuka, Fujio F. Fujiko and Amin Oka da.