NOTE: These notes are by Allan Gottlieb, and are reproduced here, with superficial modifications, with his permission. "I" in this text generally refers to Prof. Gottlieb, except in regards to administrative matters.
A lower bound on performance. Any decent scheme should do better.
Replace the page whose next reference will be furthest in the future.
Divide the frames into four classes and make a random selection from the lowest nonempty class.
Assumes that in each PTE there are two extra flags R (sometimes called U, for used) and M (often called D, for dirty).
Also assumes that a page in a lower priority class is cheaper to evict.
Every k clock ticks, reset all R bits
What if the hardware doesn't set these bits?
Simple but poor since usage of the page is ignored.
Belady's Anomaly: Can have more frames yet generate more faults. Example given later.
Similar to the FIFO PRA but when time choosing a victim, if the page at the head of the queue has been referenced (R bit set), don't evict it. Instead reset R and move the page to the rear of the queue (so it looks new). The page is being a second chance.
What if all frames have been referenced?
Becomes the same as fifo (but takes longer).
Might want to turn off the R bit more often (say every k clock ticks).
Same algorithm as 2nd chance, but a better (and I would say obvious) implementation: Use a circular list.
Do an example.
This is terrible! Why?
Ans: All but the last frame are frozen once loaded so you can replace only one frame. This is especially bad after a phase shift in the program when it is using all new pages.
When a page fault occurs, choose as victim that page that has been unused for the longest time, i.e. that has been least recently used.
LRU is definitely
NFU doesn't distinguish between old references and recent ones. The following modification does distinguish.
Homework: 25, 34
The goal is to specify which pages a given process needs to have memory resident in order for the give process to run without too many page faults.
The idea of the working set policy is to ensure that each process keeps its working set in memory.
Interesting questions include:
... Various approximations to the working set, have been devised.
0. Let P be the process currently running. Records for active pages for P are kept in a circular list, as in the clock algorithm. In each record, there is an M and R bit set by hardware at each memory references, as in NRU. At each clock tick the R bit is reset to 0, as in NRU.
1. Each page record has a field for storing the time of the most recent reference. At each clock tick, the current cumulative CPU time for P is stored in the record of each page where the R bit is 1. Thus, the time field is an approximation to the time of the most recent reference, accurate to the clock period, so similar to the information needed in the LRU strategy, but does not require the imaginary hardware of having a clock time recorded at every memory reference. This is the most elegant idea in the WSClock algorithm.
2. One might think, now, that we would continue to approximate the LRU strategy by choosing the page with the earliest time field to replace. But instead, we do as follows: There is a fixed parameter Tau, and any page whose latest reference is older than Tau is considered equally a candidate for replacement. The presumption is that pages older than Tau are not in the working set. The OS designer works on tuning Tau so that this is more or less true in practice. This is where the influence of the idea of the working set comes into the algorithm.
The advantage of doing this is that you don't have to search through all the active pages to find the earliest time stamp; you can stop when you find one older than Tau.
If no pages have a reference older than Tau, then the page with the earliest time field is chosen for replacement.
3. We prefer, of course, to replace a clean page than a dirty page, because clean pages don't have to be copied out. So we search until we find a clean page that is older than Tau, if there is one; if not, we use a dirty page older than Tau.
4. On the other hand, if we encounter a dirty page that is older than Tau, then we may as well copy it out, on the presumption that it's not part of the working set, and it will have to be written out sooner or later in any case. Suppose we've decided to write out old dirty pages D1 through Dk and to replace old clean page C with new page N. Then we ask the disk driver to schedule these k writes and 1 read. We certainly have to block P until N is completely read in, but there's no need to block P to wait for the writings out of D1 through Dk. These can go on concurrently with P. (As we shall see, the order in which disk requests are taken is up to the disk driver, which has its own agenda.) Presumably, though Tanenbaum doesn't specifically say, the M bits of D1 through Dk are set to 0 as each is successfully written out.
Similarly, suppose we can't find an old clear page, so we've decided to write out old dirty pages D1 through Dk and to replace old dirty page D0 with new page N. Then P has to block until D0 has been written out and N has been read in, but does not have to block for D1 ... Dk.
5. However, we don't want to completely tie up the disk driver with this stuff, so we set a limit on the number k of dirty pages to be written out at once.
6. Finally, as in the clock algorithm, we keep the page table entries in a circular list. Each time we start searching circularly at the current value of the list pointer. As soon as we encounter an old clean page we stop. If there is not old clean page, we use an old dirty page. If there are no pages older than Tau, then we use the oldest page, clean or dirty. At the next search, we continue on from where the list pointer left off. I don't see that this circular structure buys you a whole lot, but presumably it prevents a situation where you always start by searching over the most frequently used pages. or it achieves some kind of fairness, or adds a soupcon of the FIFO strategy, or something.
That's the algorithm.
|Random||Poor, used for comparison|
|Optimal||Unimplementable, use for comparison|
|FIFO||Not good ignores frequency of use|
|Second Chance||Improvement over FIFO|
|Clock||Better (natural) implementation of Second Chance|
|LRU||Great but impractical|
|NFU||Crude LRU approximation|
|Aging||Better LRU approximation|
|Working Set||Good, but expensive|
|WSClock||Good approximation to working set|