Bellosa, Frank
:
Process Cruise Control: Throttling Memory Access in a Soft Real-Time Environment.
Erlangen:
FAU.
1997
TR-I4-97-02.- Interner Bericht.
14 Seiten.
Abstract:
The advances in memory technology concerning performance have not been
able to keep pace with those in processor technology. Processors
clocked with hundreds of megahertz exceed the speed of affordable
memory by factors. Caches can decouple the speed of the processing
unit from the speed of the memory system if applications show a high
locality of reference. Unfor tunately, operations on data
streams frequently found in soft real-time multimedia
applications do not show this benign behavior. Thus, applications
working on data streams rely heavily upon a guaranteed memory
bandwidth to meet specific timing requirements. In multiprocessor
systems the available memory bandwidth is shared by all processing
units and DMA devices. Consequently, the processing units can
interfere and slow each other down when accessing memory. Up to now,
this effect, called memory preemption, is not covered by real- time
operating systems using timer and I/O related information.
Our novel approach to resource management is based on knowledge derived from
counters in the memory subsystem. We demonstrate that the use of
information related to cache- and main- memory access opens new
dimensions of resource management in shared-memory architec tures.
The introduction of memory-bandwidth guarantees adds a further resource
to capacity- reservation models and therefore enhances the quality of
service.
Soft real-time processes can request memory bandwidth guarantees.
Processes without guaran tees are throttled, so that they do not
withhold valuable main memory bandwidth from real-time applications.
To dynamically slow down processes that exceed a certain number of
main mem ory operations per time frame, the TLB-miss handler executes
additional NOP instructions. Thus, the TLB-fill will be delayed, fewer
memory pages can be touched and fewer cache misses per time frame will
occur. This new mechanism, called Process Cruise Control, maintains the
execution speed of soft real-time applications in a multiprocessor
environment. Applications of other scheduling classes (e.g., Time-Sharing),
which operate with low memory demands, run at full speed, whereas applications
with high memory demands will be throttled in their speed of execution and
executed with lower priority. Measurements conducted on a prototype
implementation using the Solaris operating system clearly demonstrate the
benefit of the memory throttle for a video conferencing application running
in a multiprogrammed multiprocessor environment.
