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A short class with Sarah and her students. The other ways are under More.
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Let's begin by discussing how we can calculate CPU time taken per instruction. The formula combines execution cycles and memory stall cycles. Can anyone tell me what those statistics might include?
Do you mean the total cycles and how long the CPU waits?
Exactly! The CPU time formula looks like this: CPU Time = Execution Cycles + (Memory Stall Cycles × Clock Cycle Time). Remember the acronym 'EMC' for Execution, Memory, and Clock. Can anyone explain what memory stall cycles are?
They are the cycles when the CPU is waiting for data because of a miss, right?
Correct! Good job, Student_2. Now let's move into stall cycles and how we calculate those using memory accesses, miss rates, and penalties.
Now, let's take the example of a CPU running at 200 MHz. Given that the cycle time is 5 ns and the base CPI is 1.1, how would we calculate the total CPI?
We need to consider instruction execution and potentially memory stalls caused by cache misses.
Exactly! With 30% memory access required, how do we calculate the average number of stalls?
We would take 30% as data operations, and the miss rate to get the stalls!
Similarly, let's compute the average CPI by adding the base CPI with stall calculations. Does anyone have the figures for calculating those?
Let's shift focus to how effective memory access times work during paging. Why is it significant to understand page faults and their rates?
Because if the page fault rate is too high, then our access times increase significantly, right?
Exactly! Can anyone remind me of the consequences of exceeding a certain page fault rate?
It affects the effective memory access time.
Well done! Managing the page fault rate is key to keeping the performance at bay. Now, how is the effective memory access time calculated?
The time taken to read from or write to memory, vital for CPU performance.
An event when the data requested is not found in the cache, incurring additional access time.
A critical factor affecting effective memory access times in a paging system.
An example of a CPU with a base CPI of 1.1 experiencing 10% memory accesses and consequences of memory stalls resulting in increased effective CPI.
A scenario where a page fault rate exceeding a certain threshold results in substantially increased memory access time, demonstrating the relationship between fault rate and performance.
When the CPU waits, it stalls in its fate; Memory access time is what we relate.
Imagine a library (CPU) where you often ask for books (data), but sometimes the librarian (cache) says the book is not on the shelf (cache miss). Each time you wait longer, you realize how important it is to keep the most popular books close by.
Remember 'CPI - Connect, Pause, Increase' to recall that higher CPI may lead to performance pauses from stalls.
MESC - Memory Access, Execution Stall Cycles, this captures the essence of what affects CPU time.
Term
What does CPI stand for?
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Term
What is a memory stall cycle?
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Term
What does a page fault imply?
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Term
Define Miss Rate.
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Cycles per Instruction, which indicates the average number of clock cycles each instruction takes to execute.
The number of cycles during which the CPU is waiting for memory access due to cache misses.
An event that occurs when a program accesses a page not currently mapped to physical memory.
The rate at which requests for data in cache result in misses, meaning the data needs to be fetched from lower levels of memory.
The additional time taken to fetch a data block from a second-level cache or main memory after a miss.
The average time taken to access memory, including both hits and misses, typically influenced by miss rates and penalties.
This section discusses how memory access time affects CPU performance, emphasizing the role of cache hits, misses, and page faults in instruction execution time.
The section illustrates the relationship between memory access times and CPU instruction execution, detailing the impact of cache hits and misses as well as page faults on overall performance. It uses examples to demonstrate calculations for cycles per instruction based on various miss rates and penalties.
This section highlights the critical relationship between memory access time and CPU performance, specifically focusing on how it affects instruction execution times. The text begins with the basic formula for CPU time per instruction, which consists of the execution cycles and memory stall cycles. The CPU time can be calculated as:
Memory stall cycles occur when the CPU has to wait for data or instructions, which can be calculated as:
To elucidate this, the section provides an example of a processor running at 200 MHz with a cycle time of 5 ns and a base cycles per instruction (CPI) of 1.1. With various instruction percentages and miss rates, we see how effective CPI increases due to memory stalls.
The section also discusses effective memory access times in paging systems, clarifying how page fault rates influence performance. It provides a scenario where the memory access time was deemed acceptable if the page fault rate remained below a threshold, illustrating the importance of managing these rates effectively to maintain CPU performance.
In summary, the effects of memory access times on CPU performance are profound. Higher miss rates lead to increased stall cycles, which can degrade performance significantly. Thus, optimizing memory access is crucial in CPU instruction processing.