* Performance Equation [#t9b6c6f0]

** CPU Performance [#r2e47a89]

CENTER:&size(20){CPU Time = Instruction Count * Clocks Per Instruction * Clock Time};


CENTER:&size(20){CPU Time = Instruction Count * Clocks Per Instruction * 1 / Clock Frequency};

*** CPU Time [#o4780140]

- Sec. / Program
- How many seconds does a program take to get completed?
- Measure of CPU performance

*** Instruction Count (IC) [#zd1c2146]

- Instructions / Program
- How many instructions are there in one program? Recall that when a program gets assembled, it is broken down into simple instructions.
- One can decrease IC by replacing each instructions with a more complex one, but that could lead to an increase in Clocks Per Instruction or/and Clock Time.
- IC of CISC processer is two or three times less than that of RISC processer, since each instructions are more complex and therefore decreases IC. 
-- Some CISC processors have iteration by default, decreasing IC to by an order of 10^2 when compared to that of RISC.
-- Note that Clocks Per Instruction may increase in a CISC processor, since each instruction will take more time to be completed.

*** Clocks Per Instruction (CPI) [#l7699705]

- Clocks / Instructions
- In a order of 10^0
- How many clocks does take to complete one instruction?
> "Number of cycles to execute some code divided by number of instructions. This is approximately the average number of cycles between instruction initiations (instruction starts)."

(cited from http://www.ece.lsu.edu/ee4720/2012/lsli02.pdf)

- CPI increases with complex intructions
- CPI can decrease with instruction-level parallelism and pipelining (overlapping execution of instructions, start out of order)


When one instruction is executed at a time, CPI is high (in this case, 4). Taken from http://www.ece.lsu.edu/ee4720/2012/lsli02.pdf


When multiple instructions are executed "out of order", or pipelined, CPI decreases significantly (in this case, 1)

- If one wants more, superscalar pipelining can decrease CPI to a level of less than 1 CPI

We can even run multiple instructions at the same tick, which is called superscalar pipelining, and this decreases CPI even more (in this case, 1/2)

*** Clock Time (CT), 1 / Clock Frequency (1/Φ) [#s6a22a7a]

- Seconds / Clocks
- In an order of 10^-9; A 2 GHz processor has a CT of 0.5ns.
- How many clocks can a processor handle in a second?
- Where Moore's law works
> "Technology has improved so that about every 3 years, linear dimensions have decreased by a factor of 2. Transistor power consumption has decreased by a similar factor. Speed increased by a similar factor until about 2005."

(cited from https://www.d.umn.edu/~gshute/arch/performance-equation.xhtml)

*** Example Problem [#y0842fb6]

- If a program that has 10^9 instructions runs on a 2 GHz processor optimized to 1 CPI, how much CPU time is required to complete the program?
    10^9 * 1 * 1/(2*10^9) = 0.5s
- If one program has 10^8 instructions and runs under 1 GHz CT and the other program has 3*10^8 instructions and runs under 2 GHZ CT, how much CPU time is required to complete both programs? (CPI is 2 for both programs)
    10^8 * 1 * 1/(1 * 10^9) + 3 * 10^8 * 2 * 1/(2 * 10^9) = 1/10 + 3/10 = 0.4s

*** References [#c8378092]
- http://web.sfc.keio.ac.jp/~rdv/computer-architecture-2018-wiki/index.php?- https://www.d.umn.edu/~gshute/arch/performance-equation.xhtml
- http://web.sfc.keio.ac.jp/~rdv/computer-architecture-2018-wiki/index.php?
- https://www.d.umn.edu/~gshute/arch/performance-equation.xhtml
- http://www0.cs.ucl.ac.uk/teaching/B261/Slides/lecture2/tsld015.htm
- http://www.ece.lsu.edu/ee4720/2012/lsli02.pdf

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