How to stop a SAP Runaway Train

"Runaway train, running right off the track." AC/DC

In my many years managing SAP infrastructures, I was regularly annoyed when ‘runaway’ user sessions clogged-up valuable and precious dialog work processes. At their worst, these runaway sessions could completely lock SAP Application Servers, slow the system for other users, and even cause outages, resulting in a severe impact to the business.

The main culprit was often developers inserting regular COMMIT WORK statements to reset the timer behind parameter rdisp/max_wprun_time and keep their programs running beyond the alert threshold.

To mitigate this, SAP has sneaked in a handful of new parameters into kernel 7.40 to enhance our control over work process runtimes. In doing so, SAP has de-emphasised rdisp/max_wprun_time and replaced it with the following parameters to specifically manage maximum runtimes for high, normal and low priority processes and prevent sessions from monopolising valuable work processes.

rdisp/scheduler/prio_low/max_runtime (Default - empty)

rdisp/scheduler/prio_normal/max_runtime (Default - 60m)

rdisp/scheduler/prio_high/max_runtime (Default - 10m)

The default values for maximum runtime depend on the priority class - the higher the priority, the more precious SAP considers the work process.

I suspect SAP introduced these new parameters to mitigate the risk to HANA-based systems becoming susceptible to failure from runaway processes causing problems like holding locks or high memory and CPU consumption. Thus I’d advise these parameters are set ahead of any HANA migration yo may be considering.

Please be sure that when you deploy the new parameters, you also remove rdisp/max_wprun_time, since the old parameter, if configured explicitly, overwrites the more specific settings. This is detailed in Note 25528 https://launchpad.support.sap.com/#/notes/25528

I’d also recommend note 2129291 (https://launchpad.support.sap.com/#/notes/2129291), which covers all changes to the kernel scheduler that impact work process quotas from SAP kernel 7.40.