Assessing and Understanding Computer Performance Performance Metrics and CPU Execution

What three hardware-related factors determine the performance of a piece of software on a specific computer?

Instruction usage, hardware implementation of instructions, and the performance of memory and I/O systems.

In the context of performance metrics, what is the 'Purchasing perspective' primarily concerned with?

Identifying which machine has the best performance, the least cost, or the best cost-to-performance ratio.

What is the 'Design perspective' primarily concerned with regarding performance metrics?

Determining which design options offer the best performance improvement for the least cost.

Term: Response Time

Definition: The total time between the start and the completion of a specific task, also known as execution time.

Term: Throughput

Definition: The total amount of work done by a computer system in a given amount of time, also known as bandwidth.

How does adding a new machine to a computer lab affect performance parameters?

It increases the throughput of the lab without necessarily decreasing the response time of individual jobs.

How does upgrading a machine with a faster processor typically affect performance parameters?

It decreases the response time (execution time) for tasks.

What is the formula relating Clock Cycle ($CC$) to Clock Rate ($CR$)?

$CC = \frac{1}{CR}$

If a processor has a 4 GHz clock rate, what is its clock cycle time in picoseconds?

250 ps

A clock rate of 2 GHz corresponds to a clock cycle time of _____.

500 ps

A clock rate of 500 MHz corresponds to a clock cycle time of _____.

2 ns

Scientific interval: Picosecond

The name for one trillionth of a second ($10^{-12}$ seconds).

Scientific interval: Nanosecond

The name for one billionth of a second ($10^{-9}$ seconds).

Scientific interval: Femtosecond

The name for one quadrillionth of a second ($10^{-15}$ seconds).

Scientific interval: Attosecond

The name for one quintillionth of a second ($10^{-18}$ seconds).

Why is the assumption that the number of clock cycles equals the number of instructions executed usually incorrect?

Different instructions (e.g., multiplication vs. addition) require different amounts of time and cycles to complete.

Which operation generally takes longer to execute: integer operations or floating-point operations?

Floating-point operations

How does accessing memory compare to accessing registers in terms of time cost?

Accessing memory takes significantly more time than accessing registers.

Term: CPU Execution Time

Definition: The time the CPU spends specifically working on a task, excluding time waiting for I/O or running other programs.

What components are included in 'Elapsed Time' (wall clock time)?

Everything, including disk accesses, memory accesses, I/O activities, and operating system overhead.

User CPU time is specifically defined as the time spent _____.

Executing the lines of code that are in the user's program.

Mathematically, how is Performance related to Execution Time?

$Performance = \frac{1}{Execution\ Time}$

If computer X is $n$ times faster than computer Y, what is the ratio of their execution times?

$\frac{Execution\ time_Y}{Execution\ time_X} = n$

Computer A runs a program in 10 seconds and Computer B runs it in 15 seconds. How much faster is A than B?

1.5 times faster

What are the three primary ways to improve CPU performance based on the execution time equation?

Decrease instruction count, decrease cycles per instruction (CPI), or increase the clock rate.

Term: Clock Cycles per Instruction (CPI)

Definition: The average number of clock cycles each instruction takes to execute in a specific program or workload.

What is the formula for the total number of CPU clock cycles required for a program?

$CPU\ clock\ cycles = Instruction\ Count \times CPI$

What is the formula for the overall effective CPI of a program with multiple instruction classes?

$\sum_{i=1}^{n} (CPI_i \times IC_i)$, where $IC_i$ is the percentage of instructions of class $i$.

Term: Instruction Mix

Definition: A measure of the dynamic frequency of different types of instructions across one or more programs.

What is the basic CPU Performance Equation for calculating execution time?

$CPU\ time = Instruction\ Count \times CPI \times Clock\ Cycle\ Time$

In the performance equation, how is Clock Rate used as an alternative to Clock Cycle Time?

$CPU\ time = \frac{Instruction\ Count \times CPI}{Clock\ Rate}$

How can a better data cache improve computer performance without changing the clock rate?

By reducing the average number of clock cycles required for load instructions (decreasing effective CPI).

How does branch prediction technology contribute to processor performance?

It reduces the average cycles required for branch instructions, thereby lowering the overall CPI.

What is the primary indicator of performance in the SPEC (System Performance Evaluation Cooperative) benchmarks?

A set of real programs and inputs agreed upon by companies to represent actual workloads.

Term: Arithmetic Mean (AM)

Definition: The average of execution times used in performance workloads, calculated as $AM = \frac{1}{n} \sum_{i=1}^{n} Time_i$.

What does a smaller Arithmetic Mean indicate regarding computer performance?

A smaller average execution time for the programs in the workload.

State the equation for Amdahl's Law regarding execution time after an improvement.

$Execution\ time_{after} = \frac{Execution\ time_{affected}}{Amount\ of\ improvement} + Execution\ time_{unaffected}$

According to Amdahl's Law, the performance enhancement of an improved feature is limited by _____.

The amount that the improved feature is actually used.

If a processor improvement makes a specific task 5 times faster, but that task only represents 20% of the execution time, why is the overall speedup small?

Because 80% of the execution time remains unaffected by the improvement.

What unit of measurement corresponds to the prefix 'Mega' ($M$)?

$10^6$ (one million)

What unit of measurement corresponds to the prefix 'Giga' ($G$)?

$10^9$ (one billion)

What unit of measurement corresponds to the prefix 'Tera' ($T$)?

$10^{12}$ (one trillion)

What unit of measurement corresponds to the prefix 'Peta' ($P$)?

$10^{15}$ (one quadrillion)

What unit of measurement corresponds to the prefix 'Exa' ($E$)?

$10^{18}$ (one quintillion)

What unit of measurement corresponds to the prefix 'Milli' ($m$)?

$10^{-3}$ (one thousandth)

What unit of measurement corresponds to the prefix 'Micro' ($\mu$)?

$10^{-6}$ (one millionth)

A clock period of 250 ps corresponds to a frequency of _____.

4 GHz

A clock period of 200 ps corresponds to a frequency of _____.

5 GHz

A clock period of 10 ns corresponds to a frequency of _____.

100 MHz

CPU time is broken down into which two main categories?

System time and user time.

Why is 'Elapsed Time' often unsuitable for comparing the specific performance of two different processors?

It is influenced by non-processor factors like I/O speed and the presence of other running programs.

To calculate the CPI of a specific program, you divide the _____ by the _____.

Total CPU clock cycles; Instruction Count.

In the Performance Equation, what does 'Instruction Count' represent?

The total number of instructions executed for the program.

If a CPU spends 50% of its time on ALU operations with a CPI of 1, what is the ALU's contribution to the total effective CPI?

0.5

If Load operations make up 20% of instructions and have a CPI of 5, what is their contribution to the total effective CPI?

1

If Code Sequence 1 uses 5 instructions and 10 cycles, while Sequence 2 uses 6 instructions and 9 cycles, which sequence is faster?

Sequence 2

How can executing two ALU instructions at once improve performance?

It effectively reduces the CPI for ALU operations, which lowers the total execution time.

What is the 'Zeptosecond' equivalent in seconds?

$10^{-21}$ seconds (one sextillionth of a second).

Concept: Latency

Definition: A synonym for response time or execution time, measuring how long it takes for a single job to run.

What is the fundamental unit of measurement for CPU execution time?

Seconds per program.

What is the unit of measurement for CPI?

Average number of clock cycles per instruction.

What is the unit of measurement for Clock Cycle Time?

Seconds per clock cycle.

According to the instruction performance slide, execution time equals the number of instructions executed multiplied by the _____.

Average time per instruction.

What is the name for $10^{-24}$ seconds?

Yoctosecond

What is the name for $10^{24}$?

Yotta

What is the name for $10^{21}$?

Zetta

What is the name for $10^3$?

Kilo

If the clock rate is doubled and the CPI and instruction count remain the same, the CPU time is _____.

Halved

The dynamic frequency of instructions across a workload is known as the _____.

Instruction mix

What is the primary goal of using Amdahl's Law in computer architecture?

To predict the overall performance gain from improving a specific sub-component of a system.