RHEL-RT GettingStarted
From RHEL-RT
Contents |
Getting Started
There is a lot of information contained in this HOWTO, which may make it difficult to identify where to start. In addition to this page, the following highlights the main sub-sections we wish to call particular attention to:
- RHEL-RT GeneralSystemTuningHowto#Try_these_first - most fruitful general system tuning tips
- RHEL-RT RealtimeSystemTuningHowto#Try_these_first - most fruitful realtime system tuning tips
- RHEL-RT SchedPrioHowto - setting realtime application scheduler priority
- RHEL-RT RealtimeSystemTuningHowto#Realtime_Performance_Monitoring_Tools - latency tracer - a new kernel performance analysis tool
Importance of Tuning
For anyone considering evaluating the performance benefits of the realtime kernel, it is crucial to understand 2 points:
- The importance of tuning
- Establishing the right perfornamce expectations
Both of these points are discussed in following Red Hat blog article: http://www.press.redhat.com/2007/09/24/building-an-informed-realtime-customer-base-at-high-performance-on-wall-street/
To quickly recap that info:
- kernel system tuning - gets you the vast majority of the improvement in determinism. For example, in many workloads, through proper system tuning the consistency of results is improved by 90%. This is why we typically recommend that customers first perform the suggested system tuning of standard RHEL to see if it satisfies their objectives.
The importance of system tuning is just as important when using the realtime kernel as standard RHEL. In fact, if you simply took an untuned system running standard RHEL and plopped in the MRG Realtime kernel, you are unlikely to notice any benefit at all. Why? Because as mentioned above, the standard tuning gets you 90% of the determinism improvement, and the realtime kernel yields that last 10% of determinism required by the most demanding workloads.
Establishing the right performance expectations - refers to the fact that the realtime kernel is not a panacea. Its objective is consistent, low-latency determinism = predictable response times. There is some additional kernel overhead associated with the realtime kernel (primarily due to handing interrupts in separately scheduled softirq threads). This increased overhead in some workloads results in some degradation in overall throughput. The exact amount is very workload dependent, ranging from 0% to 30%. It is the cost of determinism.
Preqequisite System Install
MRG Realtime is not a fully installable iso set. Rather it is primarily a kernel replacement. Hence there is a prerequisite system install of RHEL5.1. The original shipping version of RHEL5.0 does not include the full set of prerequisite packages.
After installing the system, you should perform the tips outlined in the tuning section.
Download the bits
Follow the download instructions on the main page to get the RPM packages. Then run your application and compare the results. In addition to the standard RHEL performance monitoring tools, keep in mind the new latency tracer utility which is described in the HOWTO. The latency tracer is useful in identifying the longest running non-preemptable kernel codepath. This information is useful in diagnosing whether non-deterministic results are attributable to the kernel or user space code.
Problem Discussion
If you have questions, the mail lists described on the main page are a great starting point. Additionally, it is described there how to enter bug reports.
