Operating Systems: AIX, HP-UX, Linux, Solaris, Windows, z/OS
Memory overload protection
Memory overload protection (MOP) limits the rate at which the ODR forwards traffic in order to prevent an OutOfMemoryException from occurring in an application server.
If traffic without server affinity arrives at the ODR and the rate for all potential servers has been exceeded, the traffic is rejected. MOP does not reject traffic that has server affinity. For example, HTTP requests with session affinity or SIP in-dialog messages.
To protect against memory overload, MOP must initially discover the maximum rate (calls per second) that can be sustained without exceeding the maximum percentage of the maximum heap size. As it is discovering the maximum rate, MOP slowly allows more traffic through without affinity, but will reject the remainder.
Initially, a potentially large number of HTTP requests or SIP messages without affinity are rejected with a 503 (unless the error code is changed). WebSphere Virtual Enterprise persists the maximum rate across server restarts, so it needs to learn the maximum rate once. The maximum rate can change over time due to changes in the session or dialog lifetimes, but these lifetimes generally change relatively slowly and MOP is able to react to such changes.
To discover the maximum rate, WebSphere Virtual Enterprise must keep the rate relatively steady for at least an averaging window. The averaging window must be at least as long as the lifetime of most of the HTTP sessions, SIP dialogs, or application sessions. Therefore, the longer the averaging window, the longer it will take to initialize.
WebSphere eXtreme Scale considerations
WebSphere eXtreme Scale might allocate additional memory in a running application server when another application server starts or stops. MOP does not currently control this memory allocation. So, if the memory utilization is already high, the additional uncontrolled allocation of memory can cause an OutOfMemoryException to occur. For example, if the maximum percentage memory setting is 90% and the current heap utilization is near 90% in application server AS1, and application server AS2 starts or stops, an OutOfMemoryException might occur in AS1 due to replication to AS1.
The maximum heap percentage should be set low enough so that there is always enough memory in reserve for the potential replication needs when an application server starts or stops. MOP will prevent an OutOfMemoryException in a dynamic cluster if the maximum percentage memory setting is set to 56%.
Related concepts
Overview of request flow prioritization
Related tasks
Configure memory overload protection
Related reference
Autonomic request flow manager cell-wide custom properties