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Windows 2000 Disk io performance Leonard Chung
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| bet | 9/47 | | Sana | 30.03.2021 | | Hajmi | 1,33 Mb. | | #13789 |
DMA vs. PIO
IDE controllers have two protocols to transfer data to and from the disk to main memory: direct memory access (DMA) and programmed input/output (PIO). The main difference between them is that while PIO requires the processor to poll the drive repeatedly and places the burden of copying the data on the processor, DMA allows the processor to just initiate a transfer and places the burden of copying the data on the controller. While the controller is copying the bits, the processor can perform other tasks. PIO is 85 times more expensive than DMA – that’s 8,500%!
DMA and PIO can be selected through the Device Manager in Win2K. We connected one IDE drive to an IDE channel on the motherboard and measured the affects of DMA and PIO. Table 7 shows the dramatic performance penalty of PIO. Not only did throughput decrease by 75% for unbuffered 64KB reads, but also processor utilization jumped to 50% across both processors. The first step to good IDE performance is to ensure that DMA is enabled. Since the 3ware IDE RAID card masquerades as a SCSI adapter, DMA and PIO selection was not necessary.
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