Finding Memory System Latency: Interleaving Effects

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Discussion Overview

The discussion revolves around understanding how to calculate the latency of a memory system, particularly in the context of memory interleaving and its effects on access time. Participants explore the relationship between bandwidth, precharge time, and data transfer size, while also addressing the applicability of certain latency equations.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant seeks an equation or explanation for calculating memory system latency given bandwidth, precharge time, and data transfer size.
  • Another participant suggests that the architecture of the memory significantly influences latency, noting that serially architected memory may exhibit long latency but high bandwidth, while random access memory typically has smaller latency.
  • A participant presents a specific homework question involving a memory system with defined parameters, expressing uncertainty about how to apply a general latency equation to the problem.
  • One participant points out that the latency equation mentioned is applicable to hard drives rather than system memory.
  • Another participant inquires if there is a specific formula for the latency of system memory, seeking clarification on the appropriate equations to use.

Areas of Agreement / Disagreement

Participants do not reach a consensus on a specific formula for calculating memory latency, and there are competing views regarding the applicability of certain equations to system memory versus hard drives.

Contextual Notes

There is uncertainty regarding the correct application of latency equations to memory systems, and participants have not resolved the differences in understanding the effects of memory architecture on latency.

Knowledge92
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I've been searching for some type of equation or explanation of how to find the latency (access time) of a memory system when given the bandwidth, the precharge (refreshing) time between accesses, and how many bytes of data are being transferred...

Also I am not sure as to how memory interleaving would change this latency...

Any help is greatly appreciated!
 
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It depends on the architecture of the memory, so I don't think there is a general relation. A serially architected memory can have a very long latency, but then a very high bandwidth once you've arrived at the correct location, while a random access memory will have a much smaller latency.
 


Thanks for that information, but maybe it'd help if I just posted my homework question:

Assume a memory system has a bandwidth of 120 MBytes (M = 2^20) per second and has aprecharge (refreshing) of 5 ns between accesses. What is the memory systems latency (access time) for the following two cases?

a. Two bytes of data are transferred and memory interleaving is not used.

b. 4-way memory interleaving is used and each module is able to transfer two bytes per access.

I am unsure how to start this. I found a computer engineering equation that latency = seek time + rotational delay + transfer time + controller delay, but I have no idea how to apply this general form of a latency equation (or if I am supposed to use this equation at all) to this type of problem.

Thank you very much!
 


Knowledge92 said:
latency = seek time + rotational delay + transfer time + controller delay
This is a formula used for hard drives, not system memory.
 


rcgldr said:
This is a formula used for hard drives, not system memory.

OK.. is there a formula for the latency of system memory/what is it?
 

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