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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Hard Disk Performance | Hard Disk External Performance Factors | Disk Interface Factors ]

CPU Utilization

Whenever a hard disk is transferring data over the interface to the rest of the system, it uses some of the system's resources. One of the more critical of these resources is how much CPU time is required for the transfer. This is called the CPU utilization of the transfer. CPU utilization is important because the higher the percentage of the CPU used by the data transfer, the less power the CPU can devote to other tasks. When multitasking, too high a CPU utilization can cause slowdowns in other tasks when doing large data transfers. Of course, if you are only doing a large file copy or similar disk access, then CPU utilization is less important.

CPU utilization is usually highest when running an IDE/ATA hard disk using a programmed I/O mode, and lower when using a DMA mode. Most newer systems use DMA modes for hard disk data transfer, provided that the appropriate hardware and drivers are installed. SCSI drives use a dedicated controller (host adapter) and typically also have low CPU utilization.

CPU utilization is one of those performance factors that is both grossly underrated and overrated at the same time. :^) Most people have never even heard of it; it often seems though that a big percentage of those who do understand its role worry about it way too much. :^) Like most performance issues, sweating small differences in numbers is usually pointless; it doesn't matter much if your CPU utilization is 5% or 10%; but if it is 80% or 90% then you are going to see an impact on the usability of the system if you multitask.

Another key issue is that faster drives transfer more data, and more data--all else being equal--requires more processing time. It's totally invalid to compare the CPU utilization of drives of different generations without correcting for this very important consideration.

One hard disk utility commonly employed for testing CPU utilization is HD Tach. (Note that this should be considered as information, not an endorsement!)

Next: Command Overhead and Multiple Device Considerations

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