CSC/ECE 506 Fall 2007/wiki1 1.3.3 1.3.4 chase2007
Introduction to Parallel Computer Architecture ->Fundamental Design Issues -> Performance (section 1.3.4)
What is Computer Performmace?
Computer performance is a measure of the out put of a computer with with respect to time and resources used.
Performance metrics
Following are the important metrics used to measure a computer's performance:
1.Latency: The time taken to perform an operation 2.Bandwidth:The rate at which the operations are performed 3.Cost: The impact these operations have on the execution time of the program
All the above metrics can be used to define a uniprocessor systems where a single CPU operates.
However, in the context of parallel computers, it becomes difficult to express the performance in above atated metrics. The reason for this is the communication between the processors that occurs mostly in the form of data transfers between the processors. So, to completely define the performace of a parallel computer, the following metrics are also considered.
1.Data Transfer Time:
It is the time taken for initiation of a data transfer and the time required for actual data transfer.So the Data Transfer Time can be given as:
Transfer Time (n) = <math>T_0+\frac{n}{B}</math>
where n = Amount of Data (in bytes)
B = Transfer Rate of the component moving the data (bytes per second)
<math>T_0</math> = Start up cost, a constant
2.Overhead and Occupancy:
The data transfer operations are initiated by the processor through communication assist
The overhead is the time the processor spends initiating the transfer of data. This may be a fixed cost, if the processor imply has to tell the communication assist to start. The overhead can also be linear with Tranfer time, if the processor has to copy the data into the assist.
The occupancy is the time it takes for the data to pass through the slowest componant on the communication path. The occupancy limits how frequently communication operations can be initiated.The next data transfer will have to wait untill the critical resource is no longer occupied before it can use the same resource.
3.Communication Cost:
It is the time that the processor spends in communicating with other processors. It can be given by the following:
Communication Cost = Frequency of Communication * (Communication Time - Overlap)
Frequency of Communication = Number of communications per unit of work
Communication Time = Overhead + Occupancy + Network Delay
Overlap = The portion of the communication operation that is performed concurrently with other useful work.