5.1 KiB


This simulator, paging-policy.py, allows you to play around with different page-replacement policies. For example, let's examine how LRU performs with a series of page references with a cache of size 3:

  0 1 2 0 1 3 0 3 1 2 1

To do so, run the simulator as follows:

prompt> ./paging-policy.py --addresses=0,1,2,0,1,3,0,3,1,2,1 
                           --policy=LRU --cachesize=3 -c

And what you would see is:

ARG addresses 0,1,2,0,1,3,0,3,1,2,1
ARG numaddrs 10
ARG policy LRU
ARG cachesize 3
ARG maxpage 10
ARG seed 0


Access: 0 MISS LRU->      [br 0]<-MRU Replace:- [br Hits:0 Misses:1]
Access: 1 MISS LRU->   [br 0, 1]<-MRU Replace:- [br Hits:0 Misses:2]
Access: 2 MISS LRU->[br 0, 1, 2]<-MRU Replace:- [br Hits:0 Misses:3]
Access: 0 HIT  LRU->[br 1, 2, 0]<-MRU Replace:- [br Hits:1 Misses:3]
Access: 1 HIT  LRU->[br 2, 0, 1]<-MRU Replace:- [br Hits:2 Misses:3]
Access: 3 MISS LRU->[br 0, 1, 3]<-MRU Replace:2 [br Hits:2 Misses:4]
Access: 0 HIT  LRU->[br 1, 3, 0]<-MRU Replace:2 [br Hits:3 Misses:4]
Access: 3 HIT  LRU->[br 1, 0, 3]<-MRU Replace:2 [br Hits:4 Misses:4]
Access: 1 HIT  LRU->[br 0, 3, 1]<-MRU Replace:2 [br Hits:5 Misses:4]
Access: 2 MISS LRU->[br 3, 1, 2]<-MRU Replace:0 [br Hits:5 Misses:5]
Access: 1 HIT  LRU->[br 3, 2, 1]<-MRU Replace:0 [br Hits:6 Misses:5]

The complete set of possible arguments for paging-policy is listed on the following page, and includes a number of options for varying the policy, how addresses are specified/generated, and other important parameters such as the size of the cache.

prompt> ./paging-policy.py --help
Usage: paging-policy.py [options]

-h, --help      show this help message and exit
                a set of comma-separated pages to access; 
                -1 means randomly generate
                a file with a bunch of addresses in it
-n NUMADDRS, --numaddrs=NUMADDRS
                if -a (--addresses) is -1, this is the 
                number of addrs to generate
-p POLICY, --policy=POLICY
                replacement policy: FIFO, LRU, LFU, OPT, 
                                    UNOPT, RAND, CLOCK
                for CLOCK policy, how many clock bits to use
                size of the page cache, in pages
-m MAXPAGE, --maxpage=MAXPAGE
                if randomly generating page accesses, 
                this is the max page number
-s SEED, --seed=SEED  random number seed
-N, --notrace   do not print out a detailed trace
-c, --compute   compute answers for me

As usual, "-c" is used to solve a particular problem, whereas without it, the accesses are just listed (and the program does not tell you whether or not a particular access is a hit or miss).

To generate a random problem, instead of using "-a/--addresses" to pass in some page references, you can instead pass in "-n/--numaddrs" as the number of addresses the program should randomly generate, with "-s/--seed" used to specify a different random seed. For example:

prompt> ./paging-policy.py -s 10 -n 3
.. .
Assuming a replacement policy of FIFO, and a cache of size 3 pages,
figure out whether each of the following page references hit or miss
in the page cache.
Access: 5  Hit/Miss?  State of Memory?
Access: 4  Hit/Miss?  State of Memory?
Access: 5  Hit/Miss?  State of Memory?

As you can see, in this example, we specify "-n 3" which means the program should generate 3 random page references, which it does: 5, 7, and 5. The random seed is also specified (10), which is what gets us those particular numbers. After working this out yourself, have the program solve the problem for you by passing in the same arguments but with "-c" (showing just the relevant part here):

prompt> ./paging-policy.py -s 10 -n 3 -c

Access: 5 MISS FirstIn->   [br 5] <-Lastin Replace:- [br Hits:0 Misses:1]
Access: 4 MISS FirstIn->[br 5, 4] <-Lastin Replace:- [br Hits:0 Misses:2]
Access: 5 HIT  FirstIn->[br 5, 4] <-Lastin Replace:- [br Hits:1 Misses:2]

The default policy is FIFO, though others are available, including LRU, MRU, OPT (the optimal replacement policy, which peeks into the future to see what is best to replace), UNOPT (which is the pessimal replacement), RAND (which does random replacement), and CLOCK (which does the clock algorithm). The CLOCK algorithm also takes another argument (-b), which states how many bits should be kept per page; the more clock bits there are, the better the algorithm should be at determining which pages to keep in memory.

Other options include: "-C/--cachesize" which changes the size of the page cache; "-m/--maxpage" which is the largest page number that will be used if the simulator is generating references for you; and "-f/--addressfile" which lets you specify a file with addresses in them, in case you wish to get traces from a real application or otherwise use a long trace as input.

One last piece of fun: why are these two examples interesting?

./paging-policy.py -C 3 -a 1,2,3,4,1,2,5,1,2,3,4,5


./paging-policy.py -C 4 -a 1,2,3,4,1,2,5,1,2,3,4,5