- A.30ms
- B.36ms
- C.54ms
- D.60ms
正确答案及解析
正确答案
解析
因为系统使用的是单缓冲区，且顺序处理9个记录，每个记录处理时间为3ms，加上读写时间，总的时间就超过3ms了。而磁盘旋转一圈的时间为27ms，也就是说，当系统读取第0个记录后，正在处理的过程中，磁盘已经旋过了第1个记录。那么，要读取第1个记录，就需要磁盘再次旋转到第1个记录（即磁盘旋转1圈后，27+3=30ms）。同理，要读取第2个记录时，也需要等30ms。这样，要读取后面8个记录，需要8*30=240ms，同时加上处理第8个记录的时间（3ms），共需243ms。
要想节约时间，可以把记录错开存放，如表2-7所示。
表2-7 错开存放的记录
这样，就可以在磁盘旋转2圈内完成所有记录的处理，时间为54ms。要注意的是，最后处理的记录R8不是最后一个磁盘块，所以不需要旋转到最后1个物理块。也就是说，第2圈的旋转时间只需要24ms就到达R8了。但是，因为要加上R8的处理时间3ms，所以，总时间仍然为54ms。
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