Measuring Results

Measurements are shown in Figure 4. Restricting the size of the page cache shows the expected behavior: The applications are not much slower, even with massively parallel I/O. The kernel patch by Mel Gorman behaves similarly: It is already available in SLES 11 SP 3 and is going to be included in RHEL 7. It supports good, consistent performance of the applications.

Figure 4: Measurements for evaluating kernel adjustments (statistical mean values for several test cycles are shown).

Also, setting swappiness = 0 on SLES 11 SP 2 and SP 3 seems to protect the applications adequately. Amazingly, Red Hat Enterprise Linux 6.4 behaves differently: The implementation seems to be substantially different and does not protect applications against aggressive swapping – on the contrary.

The different values for swappiness do not show any clear trend. Although they lead to a clearly poorer performance of the application with increasing I/O, it is difficult to distinguish systematically between smaller values like 10 or 30 and the default value of 60. It seems that the crucial question is whether or not swappiness is set to  . Intermediate values have hardly any effect.

Table 1 summarizes all the approaches and gives an overview of their advantages and disadvantages, as well as the measured results. Currently no solution eliminates all aspects of the displacement problem. The next best thing still seems to be a correct implementation of swappiness – and possibly, in future, the approach by Mel Gorman. Even with these two methods, however, administrators of systems with large amounts of main memory will not be able to avoid keeping a watchful eye on the memory usage of their applications.

Conclusions

Displacement of applications from RAM still proves to be a problem, even with very well equipped systems. Although memory shortage should no longer be an issue with such systems, the basically sensible, intensive utilization of memory by the Linux kernel can lead to significant performance problems in applications.

Linux is in quite good shape compared with other operating systems, but it can be useful – given the variety of approaches that is typical of Linux  – to investigate the behavior of the kernel and the applications you use, so you can operate large systems with consistently high performance.

For more details on experience with these systems and the tests used in this article, check out the Test Drive provided by SAP LinuxLab on the SAP Community Network (SCN) [14].