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Assignment 4 - Cache Simulator
Computer Architecture Assignment 4 - Cache Simulator
1 Overview
The goal of this assignment is to help you understand caches better. You are required to write a cache simulator using the C programming language. The programs have to run on iLab machines. We are providing real program memory traces as input to your cache simulator. The format and structure of the memory traces are described below.
2 Memory Access Traces
The input to the cache simulator is a memory access trace, which we have generated by executing real programs. The trace contains memory addresses accessed during program execution. Your cache simulator will have to use these addresses to determine if the access is a hit or a miss, and the actions to perform in each case. The memory trace file consists of multiple lines. Each line of the trace file corresponds to a memory accesses performed by the program. Each line consists of multiple columns, which are space separated. The first column reports the PC (program counter) when this particular memory access occurred, followed by a colon(:). Second column lists whether the memory access is a read (R) or a write (W) operation. And the last column reports the actual 48-bit memory address that has been accessed by the program. In this assignment, you only need to consider the second and the third columns (i.e. you dont really need to know the PCs). The last line of the trace file will be the string #eof. We have provided you three input trace files (some of them are larger in size). Here is a sample trace file.
0x804ae19: R 0x9cb3d40 0x804ae19: W 0x9cb3d40 0x804ae1c: R 0x9cb3d44 0x804ae1c: W 0x9cb3d44 0x804ae10: R 0xbf8ef498 #eof
3 Cache Simulator
You will implement a cache simulator to evaluate different configurations of caches. It should be able to run with different traces files. The followings are the requirements for your cache simulator:
• Simulate only one level cache; i.e., an L1 cache. • The cache size, associativity, the replacement policy, and the block size are input parameters. Cache size and block size are specified in bytes. • Replacement algorithm: First In First Out (FIFO). When a block needs to be replaced, the cache evicts the block that was accessed first. It does not take into account whether the block is frequently or recently accessed. • You have to simulate a write through cache.
4 Cache Simulator Interface
You have to name your cache simulator first. Your program should support the following usage interface: ./first <cache size<associativity<cache policy<block size<trace file
where: A) <cache sizeis the total size of the cache in bytes. This number should be a power of 2. B) <associativityis one of: direct - simulate a direct mapped cache. assoc - simulate a fully associative cache. assoc:n - simulate an n way associative cache. n will be a power of 2. C) <cache policyHere is valid cache policy is fifo. D) <block sizeis a power of 2 integer that specifies the size of the cache block in bytes. E) <trace fileis the name of the trace file.
Your program should check if all the inputs are in valid format, if not print error and then terminate the program.
5 Cache Prefetcher
Prefetching is a common technique to increase the spatial locality of the caches beyond the cache line. The idea of prefetching is to bring the data into the cache before it is needed (accessed). In a normal cache, you bring a block of data into the cache whenever you experience a cache-miss. Now, we want you to explore a different type of cache that
prefetches not only brings the block corresponding to the access but also prefetches one adjacent block, which will result in one extra memory read. For example, if a memory address 0x40 misses in the cache and the block size is 4 bytes, then the prefetcher would bring the block corresponding to 0x40 + 4 into the cache. The prefetcher is activated only on misses and it is not active on a cache hit. If the prefetched block is already in the cache, it does not issue a memory read. With respect to cache replacement policies, if the prefetched block hits in the cache, the line replacement policy status should not be updated. Otherwise, it is treated similar to a block that missed the cache.
6 Cache Replacement Policy
The goal of the cache replacement policy is to decide which block has to be evicted in case there is no space in the set for an incoming cache block. It is always preferable – to achieve the best performance – to replace the block that will be re-referenced furthest in the future. There are different ways one can implement cache replacement policy. Here we use FIFO replacement policy and LRU policy as extra credit.
6.1 FIFO
Using this algorithm, you can always evict the block accessed first in the set without any regard to how often or how many times it was accessed before. So let us say that your cache is empty initially and that each set has two ways. Now suppose that you access blocks A, B, A, C. To make room for C, you would evict A since it was the first block to be brought into the set.
7 Sample Run
Your program should print out the number of memory reads (per cache block), memory writes (per cache block), cache hits, and cache misses for normal cache and the cache with prefetcher. You should follow the exact same format shown below (pay attention to case sensitivity of the letters), otherwise, the autograder can not grade your program properly. $./first 32 assoc:2 fifo 4 trace2.txt no-prefetch Memory reads: 3499 Memory writes: 2861 Cache hits: 6501 Cache misses: 3499 with-prefetch Memory reads: 3521
Memory writes: 2861 Cache hits: 8124 Cache misses: 1876
In this example above, we are simulating 2-way set associate cache of size 32 bytes. Each cache block is 4 bytes. The trace file name is trace2.txt. As you can see, the simulator should simulate both catch types with the prefetcher and without the prefetcher in a single run and display the results for both. Note: Some of the trace files are quite large. So it might take a few minutes for the autograder to grade for all the testcases.
8 Simulation Details
1. (a) When your program starts, there is nothing in the cache. So, all cache lines are empty (invalid). (b) you can assume that the memory size is 2pow48 . Therefore, memory addresses are 48 bit (zero extend the addresses in the trace file if theyre less than 48-bit in length). (c) the number of bits in the tag, cache address, and byte address are determined by the cache size and the block size;
2. For a write-through cache, there is the question of what should happen in case of a write miss. In this assignment, the assumption is that the block is first read from memory (one read memory), and then followed by a memory write. 3- You do not need to simulate the memory in this assignment. Because, the traces doesnt contain any information on data values transferred between the memory and the caches. 4. You have to compile your program with the following flags: -Wall -Werror -fsanitize=address
9 Extra credit (50 points)
As an extra credit, you should implement LRU (Least Recently Used) cache policy. Your program should output exactly the same format output as it shown before. Please note that, you should clearly mention in the report that youve done extra credit otherwise you may not get the points. Here is an example of running your program with LRU policy. $./first 32 assoc:2 lru 4 trace2.txt no-prefetch Memory reads: 3292 Memory writes: 2861 Cache hits: 6708 Cache misses: 3292
with-prefetch Memory reads: 3315 Memory writes: 2861 Cache hits: 8331 Cache misses: 1669
10 Submission
You have to e-submit the assignment using Sakai . Put all files (source code + Makefile + report.pdf) into a directory named first, which itself is a sub-directory under pa4 . Then, create a tar file (follow the instructions in the previous assignments to create the tar file). Your submission should be only a tar file named pa4.tar. You have to e-submit the assignment using Sakai. Your submission should be a tar file named pa4.tar. To create this file, put everything that you are submitting into a directory named pa4. Then, cd into the directory containing pa4 (that is, pa4s parent directory) and run the following command: $tar cvf pa4.tar pa4
To check that you have correctly created the tar file, you should copy it (pa4.tar) into an empty directory and run the following command: $tar xvf pa4.tar
This is how the folder structure should be.
• pa4 – first ∗ first.c ∗ first.h ∗ Makefile ∗ report.txt
Source code: all source code files necessary for building your programs. e.g. first.c and first.h. Makefile: There should be at least two rules in your Makefile: first: build the executables (first). clean: prepare for rebuilding from scratch.
report.txt : In a text file, you should briefly describe the main data structures being used in your program. More importantly, you should report your observation on how the prefetcher changed the cache hits and number of memory reads. Explain why?
11 Autograder
First mode Testing when you are writing code with a pa4 folder. 1. Lets say you have a pa4 folder with the directory structure as described in the assignment. 2. Copy the folder to the directory of the autograder 3. Run the autograder with the following command $python auto grader.py It will run the test cases and print your scores. Second mode This mode is to test your final submission (i.e, pa4.tar) 1. Copy pa4.tar to the autograder directory 2. Run the autograder with pa4.tar as the argument as below: $python auto grader.py pa4.tar
12 Grading guidelines
1. We should be able build your program by just running make. 2. Your program should follow the format specified above for the usage interface. 3. Your program should strictly follow the input and output specifications mentioned above. (Note: This is perhaps the most important guideline: failing to follow it might result in you losing all or most of your points for this assignment. Make sure your programs output format is exactly as specified. Any deviation will cause the automated grader to mark your output as incorrect. REQUESTS FOR RE-EVALUATIONS OF PROGRAMS REJECTED DUE TO IMPROPER FORMAT WILL NOT BE ENTERTAINED.)
1 Overview
The goal of this assignment is to help you understand caches better. You are required to write a cache simulator using the C programming language. The programs have to run on iLab machines. We are providing real program memory traces as input to your cache simulator. The format and structure of the memory traces are described below.
2 Memory Access Traces
The input to the cache simulator is a memory access trace, which we have generated by executing real programs. The trace contains memory addresses accessed during program execution. Your cache simulator will have to use these addresses to determine if the access is a hit or a miss, and the actions to perform in each case. The memory trace file consists of multiple lines. Each line of the trace file corresponds to a memory accesses performed by the program. Each line consists of multiple columns, which are space separated. The first column reports the PC (program counter) when this particular memory access occurred, followed by a colon(:). Second column lists whether the memory access is a read (R) or a write (W) operation. And the last column reports the actual 48-bit memory address that has been accessed by the program. In this assignment, you only need to consider the second and the third columns (i.e. you dont really need to know the PCs). The last line of the trace file will be the string #eof. We have provided you three input trace files (some of them are larger in size). Here is a sample trace file.
0x804ae19: R 0x9cb3d40 0x804ae19: W 0x9cb3d40 0x804ae1c: R 0x9cb3d44 0x804ae1c: W 0x9cb3d44 0x804ae10: R 0xbf8ef498 #eof
3 Cache Simulator
You will implement a cache simulator to evaluate different configurations of caches. It should be able to run with different traces files. The followings are the requirements for your cache simulator:
• Simulate only one level cache; i.e., an L1 cache. • The cache size, associativity, the replacement policy, and the block size are input parameters. Cache size and block size are specified in bytes. • Replacement algorithm: First In First Out (FIFO). When a block needs to be replaced, the cache evicts the block that was accessed first. It does not take into account whether the block is frequently or recently accessed. • You have to simulate a write through cache.
4 Cache Simulator Interface
You have to name your cache simulator first. Your program should support the following usage interface: ./first <cache size<associativity<cache policy<block size<trace file
where: A) <cache sizeis the total size of the cache in bytes. This number should be a power of 2. B) <associativityis one of: direct - simulate a direct mapped cache. assoc - simulate a fully associative cache. assoc:n - simulate an n way associative cache. n will be a power of 2. C) <cache policyHere is valid cache policy is fifo. D) <block sizeis a power of 2 integer that specifies the size of the cache block in bytes. E) <trace fileis the name of the trace file.
Your program should check if all the inputs are in valid format, if not print error and then terminate the program.
5 Cache Prefetcher
Prefetching is a common technique to increase the spatial locality of the caches beyond the cache line. The idea of prefetching is to bring the data into the cache before it is needed (accessed). In a normal cache, you bring a block of data into the cache whenever you experience a cache-miss. Now, we want you to explore a different type of cache that
prefetches not only brings the block corresponding to the access but also prefetches one adjacent block, which will result in one extra memory read. For example, if a memory address 0x40 misses in the cache and the block size is 4 bytes, then the prefetcher would bring the block corresponding to 0x40 + 4 into the cache. The prefetcher is activated only on misses and it is not active on a cache hit. If the prefetched block is already in the cache, it does not issue a memory read. With respect to cache replacement policies, if the prefetched block hits in the cache, the line replacement policy status should not be updated. Otherwise, it is treated similar to a block that missed the cache.
6 Cache Replacement Policy
The goal of the cache replacement policy is to decide which block has to be evicted in case there is no space in the set for an incoming cache block. It is always preferable – to achieve the best performance – to replace the block that will be re-referenced furthest in the future. There are different ways one can implement cache replacement policy. Here we use FIFO replacement policy and LRU policy as extra credit.
6.1 FIFO
Using this algorithm, you can always evict the block accessed first in the set without any regard to how often or how many times it was accessed before. So let us say that your cache is empty initially and that each set has two ways. Now suppose that you access blocks A, B, A, C. To make room for C, you would evict A since it was the first block to be brought into the set.
7 Sample Run
Your program should print out the number of memory reads (per cache block), memory writes (per cache block), cache hits, and cache misses for normal cache and the cache with prefetcher. You should follow the exact same format shown below (pay attention to case sensitivity of the letters), otherwise, the autograder can not grade your program properly. $./first 32 assoc:2 fifo 4 trace2.txt no-prefetch Memory reads: 3499 Memory writes: 2861 Cache hits: 6501 Cache misses: 3499 with-prefetch Memory reads: 3521
Memory writes: 2861 Cache hits: 8124 Cache misses: 1876
In this example above, we are simulating 2-way set associate cache of size 32 bytes. Each cache block is 4 bytes. The trace file name is trace2.txt. As you can see, the simulator should simulate both catch types with the prefetcher and without the prefetcher in a single run and display the results for both. Note: Some of the trace files are quite large. So it might take a few minutes for the autograder to grade for all the testcases.
8 Simulation Details
1. (a) When your program starts, there is nothing in the cache. So, all cache lines are empty (invalid). (b) you can assume that the memory size is 2pow48 . Therefore, memory addresses are 48 bit (zero extend the addresses in the trace file if theyre less than 48-bit in length). (c) the number of bits in the tag, cache address, and byte address are determined by the cache size and the block size;
2. For a write-through cache, there is the question of what should happen in case of a write miss. In this assignment, the assumption is that the block is first read from memory (one read memory), and then followed by a memory write. 3- You do not need to simulate the memory in this assignment. Because, the traces doesnt contain any information on data values transferred between the memory and the caches. 4. You have to compile your program with the following flags: -Wall -Werror -fsanitize=address
9 Extra credit (50 points)
As an extra credit, you should implement LRU (Least Recently Used) cache policy. Your program should output exactly the same format output as it shown before. Please note that, you should clearly mention in the report that youve done extra credit otherwise you may not get the points. Here is an example of running your program with LRU policy. $./first 32 assoc:2 lru 4 trace2.txt no-prefetch Memory reads: 3292 Memory writes: 2861 Cache hits: 6708 Cache misses: 3292
with-prefetch Memory reads: 3315 Memory writes: 2861 Cache hits: 8331 Cache misses: 1669
10 Submission
You have to e-submit the assignment using Sakai . Put all files (source code + Makefile + report.pdf) into a directory named first, which itself is a sub-directory under pa4 . Then, create a tar file (follow the instructions in the previous assignments to create the tar file). Your submission should be only a tar file named pa4.tar. You have to e-submit the assignment using Sakai. Your submission should be a tar file named pa4.tar. To create this file, put everything that you are submitting into a directory named pa4. Then, cd into the directory containing pa4 (that is, pa4s parent directory) and run the following command: $tar cvf pa4.tar pa4
To check that you have correctly created the tar file, you should copy it (pa4.tar) into an empty directory and run the following command: $tar xvf pa4.tar
This is how the folder structure should be.
• pa4 – first ∗ first.c ∗ first.h ∗ Makefile ∗ report.txt
Source code: all source code files necessary for building your programs. e.g. first.c and first.h. Makefile: There should be at least two rules in your Makefile: first: build the executables (first). clean: prepare for rebuilding from scratch.
report.txt : In a text file, you should briefly describe the main data structures being used in your program. More importantly, you should report your observation on how the prefetcher changed the cache hits and number of memory reads. Explain why?
11 Autograder
First mode Testing when you are writing code with a pa4 folder. 1. Lets say you have a pa4 folder with the directory structure as described in the assignment. 2. Copy the folder to the directory of the autograder 3. Run the autograder with the following command $python auto grader.py It will run the test cases and print your scores. Second mode This mode is to test your final submission (i.e, pa4.tar) 1. Copy pa4.tar to the autograder directory 2. Run the autograder with pa4.tar as the argument as below: $python auto grader.py pa4.tar
12 Grading guidelines
1. We should be able build your program by just running make. 2. Your program should follow the format specified above for the usage interface. 3. Your program should strictly follow the input and output specifications mentioned above. (Note: This is perhaps the most important guideline: failing to follow it might result in you losing all or most of your points for this assignment. Make sure your programs output format is exactly as specified. Any deviation will cause the automated grader to mark your output as incorrect. REQUESTS FOR RE-EVALUATIONS OF PROGRAMS REJECTED DUE TO IMPROPER FORMAT WILL NOT BE ENTERTAINED.)
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