operating system - About CPU operation and I/O processing -


my question why want have cpu's operation overlap of i/o processing. have been thinking optimization , such yet arrive @ conclusion.

if able answer question, great. :d

i/o slow compared operating frequency of cpu.

suppose have 1ghz cpu that's capable of executing 1 instruction every clock cycle. means cpu able execute 1 instruction every nanosecond.

now let's assume want fetch data hard drive. disk operations take place in milisecond scale, , we'll assume drives fast enough fetch data in 1ms.

if cpu sit around , wait disk fetch data, cpu waste 1 million nanoseconds doing nothing, whereas executing 1 million instructions task. when program has lot of io access, wasted cycles stacks , become noticeable if let cpu wait , nothing. why it's idea overlap computation io cpu cycles aren't wasted.

this why computer becomes super unresponsive when main memory full, , cpu has page disk. cpu cannot perform useful task unless data needs has been retrieved disk main memory, must sit around , wait ios complete.


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