Communicator e3000 MPE/iX Release 6.5 (Non-Platform Software Release C.65.00 (30216-90291)

60 Chapter3

Growth Solution

Dispatcher Multiple Runqueues

by Senthil Kumar R

Commercial Systems Division

Introduction

The MPE/iX Operating System is continuously undergoing various enhancements to keep

pace with the increasing performance needs of HP 3000 customers. The support for

Dispatcher Multiple Runqueues is one such enhancement in MPE/iX Release 6.5 which is

aimed at providing improved system performance to customers. Dispatcher is one of the

core modules of the MPE/iX OS; it is responsible for allocating CPU time to processes. This

article gives a technical overview of the changes to the Dispatcher module to provide

improved performance on 10-way and 12-way systems in MPE/iX 6.5.

Technical Overview

Dispatcher maintains the list of ready-to-run processes in the system in a queue called

“runqueue”. Earlier versions of MPE/iX used a single runqueue to maintain these

processes. This was true even in multiprocessor (MP) systems which have more than one

processor. So in an MP system all processors have to contend for this single runqueue

while picking up processes for execution. When one processor is accessing the runqueue,

other processors wanting to access the queue have to wait until the runqueue is free. This

serialization causes severe contention in the system and leads to loss of performance,

especially in higher-end multiprocessor systems.

To overcome the performance loss due to runqueue contention, MPE/iX 6.5 supports

multiple runqueues in Dispatcher. There can be as many runqueues as the number of

processors on the system. The ready processes can be enqueued to any of these runqueues.

With multiple runqueues available, each processor can pick ready processes for execution

independent of other processors. This implies that all processors can now select in parallel

ready processes for execution instead of doing it serially as in the case of a single

runqueue. The processors do not waste time waiting on the runqueue and can do more

useful work. This results in improved system throughput and performance.

Beneﬁts to Customers

The support for multiple runqueues in Dispatcher brings many advantages for MPE/iX

customers. With this feature, the scalability of MPE/iX has been considerably increased.

This means that the increase in system performance with the addition of more processors

is much more than what it was earlier. This is a key aspect to enable customers to upgrade

their HP 3000 systems with a larger number of processors to fulﬁll their performance

needs. Also the system response time for users is expected to be better since process

scheduling will be faster now.

In addition to supporting multiple runqueues, there have been other changes in the

Dispatcher module to make it more efﬁcient for execution in multiprocessor environment.

This has been done by carefully rearchitecting various synchronization mechanisms in the

implementation to cut down overheads and focus on delivering faster response time.