CS433g: Computer System Organization – Fall 2005
Practice Set (Chapter 7) – Solution
Due Date: None
Question 1
The I/O bus and a memory system of a computer are capable of sustaining 1000MB/sec
without interfering with the performance of an 800-MIPS CPU (costing $50,000). Here
are the assumptions about the database software that must run on this computer:
• Each transaction requires 2 disk reads plus 2 disk writes
• The operating system uses 15,000 instructions for each disk read or write
• The database software executes 40,000 instructions to process a transaction
• The transfer size is 1000 bytes per disk I/O
You have a choice of two types of disk organizations:
• Several small disks that store 500MB each and cost $100 each
• Several big disks that store 1250MB each and cost $250 each
Either disk system can support on average 30 disk reads or writes per second.
You may approximate 1 MB as 1,000,000 bytes and 1 GB as 1000 MB.
The system must support 10 gigabytes of data in the system. Assume that disk requests
can be spread evenly to all the disks, in a round robin fashion (i.e., I/O request 1 goes to
disk 1, request 2 goes to disk 2, etc.).
Part A
What is the maximum transactions per second (TPS) rate possible with each disk
organization? For this question, you should compute the peak TPS that would be allowed
by each system component (CPU, bus, disk) and then determine the TPS from the
limiting component.
Solution:
We need 10GB of disk space so this requires either 20 small disks or 8 large disks.
Each transaction requires (2 reads + 2 writes) * 15,000 + 40,000 = 100,000 instructions
per transaction.
Unit Performance # of Units Demand/Transaction TPS Limit
CPU 800 MIPS 1 1 MIPS 8,000
Bus 1000 MB/s 1 1000 Bytes 250,000
Small
Disk 30 I/Os per
second 20 4 I/Os 150
Large
Disk 30 I/Os per
second 8 4 I/Os 60
