601.229 (S20): Assignment 3: Cache simulator

Due: ~~Thursday, March 26th~~ Monday, March 30th by 11pm

Update 3/16: minor change to grading rubric to allocate points for gracefully handling invalid parameters

Update 3/18: clarified that loads and stores in the trace will access at most 4 bytes

Cache simulator

Acknowledgment: This assignment was originally developed by Peter Froehlich for his version of CSF.

This problem focuses on simulating and evaluating caches. We’ll give you a number of memory traces from real benchmark programs. You’ll implement a program to simulate how a variety of caches perform on these traces. You’ll then use your programs and the given traces to determine the best overall cache configuration.

Grading criteria

Your grade will be determined as follows:

Gracefully handling invalid parameters: 2%
Accurate load count: 8%
Accurate store count: 8%
Accurate load hits: 14%
Accurate load misses: 14%
Accurate store hits: 14%
Accurate store misses: 14%
Accurate total cycles: 6%
Report on best cache: 10%
Design, coding style: 10%

For the numeric results, the Total cycles output only needs to be within ±10%, while the other results must be exact.

Make sure you follow the style guidelines.

Your program should execute without memory errors or memory leaks. Memory errors such as invalid reads or write, or uses of uninitialized memory, will result in a deduction of up to 10 points. Memory leaks will result in a deduction of up to 5 points.

Programming Languages

You can use either C or C++ for this assignment. You’re allowed to use the standard library of your chosen language as much as you would like to, but you are not allowed to use any additional (non-standard) libraries.

One advantage of choosing C++ is that you can use the built-in container data structures such as map, vector, etc. (Note however that it is entirely possible to create a straightforward and robust implementation of this program using dynamically-allocated arrays.) Regardless of which language you use, we highly encourage you to write modular, well-designed code, and to develop data types and functions to manage the complexity of the program. Strive for simplicity.

You must provide a Makefile such that

make clean removes all object files and executables, and
make or make csim compiles and links your program, producing an executable called csim

Your code should compile cleanly with gcc 7.x using the -Wall -Wextra --pedantic compiler flags. Important: your Makefile must use these options. If your Makefile does not compile your code with these options, you will forfeit all of the points for design and coding style.

Part (a): Cache Simulator

You will design and implement a cache simulator that can be used to study and compare the effectiveness of various cache configurations. Your simulator will read a memory access trace from standard input, simulate what a cache based on certain parameters would do in response to these memory access patterns, and finally produce some summary statistics to standard output. Let’s start with the file format of the memory access traces:

s 0x1fffff50 1
l 0x1fffff58 1
l 0x1fffff88 6
l 0x1fffff90 2
l 0x1fffff98 2
l 0x200000e0 2
l 0x200000e8 2
l 0x200000f0 2
l 0x200000f8 2
l 0x30031f10 3
s 0x3004d960 0
s 0x3004d968 1
s 0x3004caa0 1
s 0x3004d970 1
s 0x3004d980 6
l 0x30000008 1
l 0x1fffff58 4
l 0x3004d978 4
l 0x1fffff68 4
l 0x1fffff68 2
s 0x3004d980 9
l 0x30000008 1

As you can see, each memory access performed by a program is recorded on a separate line. There are three “fields” separated by white space. The first field is either l or s depending on whether the processor is “loading” from or “storing” to memory. The second field is a 32-bit memory address given in hexadecimal; the 0x at the beginning means “the following is hexadecimal” and is not itself part of the address. You can ignore the third field for this assignment.

Note that you should assume that each load or store in the trace accesses at most 4 bytes of data, and that no load or store accesses data which spans multiple cache blocks (a.k.a. “lines”.)

Your cache simulator will be configured with the following cache design parameters which are given as command-line arguments (see below):

number of sets in the cache (a positive power-of-2)
number of blocks in each set (a positive power-of-2)
number of bytes in each block (a positive power-of-2, at least 4)
write-allocate or no-write-allocate
write-through or write-back
lru (least-recently-used) or fifo evictions

Note that certain combinations of these design parameters account for direct-mapped, set-associative, and fully associative caches:

a cache with n sets of 1 block each is direct-mapped
a cache with n sets of m blocks each is m-way set-associative
a cache with 1 set of n blocks is fully associative

The smallest cache you must be able to simulate has 1 set with 1 block with 4 bytes; this cache can only remember a single 4-byte memory reference and nothing else; it can therefore only be beneficial if consecutive memory references in a trace go to the exact same address. You should probably use this tiny cache for basic sanity testing.

A few reminders about the other three parameters: The write-allocate parameter determines what happens for a cache miss during a store:

for write-allocate we bring the relevant memory block into the cache before the store proceeds
for no-write-allocate a cache miss during a store does not modify the cache

Note that this parameter interacts with the following one. The write-through parameter determines whether a store always writes to memory immediately or not:

for write-through a store writes to the cache as well as to memory
for write-back a store writes to the cache only and marks the block dirty; if the block is evicted later, it has to be written back to memory before being replaced

It doesn’t make sense to combine no-write-allocate with write-back because we wouldn’t be able to actually write to the cache for the store!

The last parameter is only relevant for associative caches: in direct-mapped caches there is no choice for which block to evict!

for lru (least-recently-used) we evict the block that has not been accessed the longest
for fifo (first-in-first-out) we evict the block that has been in the cache the longest

Your cache simulator should assume that loads/stores from/to the cache take one processor cycle; loads/stores from/to memory take 100 processor cycles for each 4-byte quantity that is transferred. There are plenty of things about caches in real processors that you do not have to simulate, for example write buffers or smart ways to fill cache blocks; implementing all the options above correctly is already somewhat challenging, so we’ll leave it at that.

We expect to be able to run your simulator as follows:

./csim 256 4 16 write-allocate write-back lru < sometracefile

This would simulate a cache with 256 sets of 4 blocks each (aka a 4-way set-associative cache), with each block containing 16 bytes of memory; the cache performs write-allocate but no write-through (so it does write-back instead), and it evicts the least-recently-used block if it has to. (As an aside, note that this cache has a total size of 16384 bytes (16 kB) if we ignore the space needed for tags and other meta-information.)

After the simulation is complete, your cache simulator is expected to print the following summary information in exactly the format given below:

Total loads: count
Total stores: count
Load hits: count
Load misses: count
Store hits: count
Store misses: count
Total cycles: count

The count value is simply an occurrence count. As a concrete example, here is an example invocation of the program on one of the example traces, gcc.trace:

./csim 256 4 16 write-allocate write-back fifo < gcc.trace

This invocation should produce the following output:

Total loads: 318197
Total stores: 197486
Load hits: 314171
Load misses: 4026
Store hits: 188047
Store misses: 9439
Total cycles: 9845283

Note that due to slight variations in how you might reasonably interpret the simulator specification, your Total cycles value could be slightly different, but should be fairly close. For all of the other counts, your simulator’s output should exactly match the output above.

We strongly encourage you to use Piazza to post traces and simulator results, so that you can compare your results with other students’ results.

Example traces

Here are some traces you can use for testing and empirical evaluation:

gcc.trace and swim.trace are traces from real programs, so you should consider using them in your empirical evaluation.

Hints

Your simulation is only concerned with hits and misses, at no point do you need the actual data that’s stored in the cache; that’s the reason why the trace files do not contain that information in the first place.

Don’t try to implement all the options right away, start by writing a simulator that can only run direct-mapped caches with write-through and no-write-allocate. Once you have that working, extend step-by-step to make the other design parameters work. Also, sanity-check your simulator frequently with simple, hand-crafted traces for which you can still derive manually what the behavior should be.

Note that accurate cycle counting is only worth 6% of the total assignment grade. Make sure that loads and stores are modeled correctly with accurate hit and miss counts before being too concerned about counting cycles.

Part (b): Best cache

For part (b), you’ll use the memory traces as well as your simulator to determine which cache configuration has the best overall effectiveness. You should take a variety of properties into account: hit rates, miss penalties, total cache size (including overhead), etc. In your README, describe in detail what experiments you ran (and why!), what results you got (and how!), and what, in your opinion, is the best cache configuration of them all.

Credits

The memory traces above come from a similar programming assignment by Steven Swanson at the University of California, San Diego. Thank you Steven!

Submitting

Create a zipfile that has your Makefile, source and header files, and README file. All of the files should be in the top level directory of the zipfile. As an example, if your zipfile is called assign3.zip, the command unzip -l assign3.zip might produce the following output:

Archive:  assign3.zip
  Length      Date    Time    Name
---------  ---------- -----   ----
    15225  2020-02-25 12:27   main.c
      149  2020-02-25 12:27   Makefile
    12075  2020-02-25 12:28   README
---------                     -------
    27449                     3 files

Your exact output will almost certainly differ, for example, depending on how you structured your cache simulator program.

Upload your zipfile to Gradescope as Assignment3.