Operating Systems

Start Lecture #10

Remark: Lab 4 (the last lab) is assigned).

3.5 Design Issues for (Demand) Paging Systems

3.5.1 Local vs Global Allocation Policies

A local PRA is one is which a victim page is chosen among the pages of the same process that requires a new frame. That is the number of frames for each process is fixed. So LRU for a local policy means the page least recently used by this process. A global policy is one in which the choice of victim is made among all pages of all processes.

Of course we can't have a purely local policy, why?
Answer: A new process has no pages and, even if we didn't restrict the frame needed to a local one for the first page loaded, the process would remain with only one page.
Perhaps wait until a process has been running a while before restricting it to existing frames or give the process an initial allocation of frames based on the size of the executable.

In general a global policy seems to work better. For example, consider LRU. With a local policy, the local LRU page might have been more recently used than many resident pages of other processes. A global policy needs to be coupled with a good method to decide how many frames to give to each process. By the working set principle, each process should be given |w(k,t)| frames at time t, but this value is hard to calculate exactly.

If a process is given too few frames (i.e., well below |w(k,t)|), its faulting rate will rise dramatically. If this occurs for many or all the processes, the resulting situation in which the system is doing very little useful work due to the high I/O requirements for all the page faults is called thrashing.

Page Fault Frequency (PFF)

An approximation to the working set policy that is useful for determining how many frames a process needs (but not which pages) is the Page Fault Frequency algorithm.

For each process keep track of the page fault frequency, which is the number of faults divided by the number of references.
Actually, must use a window or a weighted calculation since you are interested in the recent page fault frequency.
Actually, it is too expensive to calculate the number of references so, as above, we approximate this by the amount of (virtual) time.
If the PFF is exceptionally low, free some of this processes frames (e.g., limit victim selection to this process for a while).
If the PFF is too high, allocate more frames to this process. Either
1. Raise its number of frames if using a local policy; or
2. Bar its frames from eviction (for a while) if using a global policy.
What if there are not enough frames in the entire system? That is, what if the PFF is too high for all processes?
Answer: Reduce the MPL as we now discuss.

3.5.2 Load Control

To reduce the overall memory pressure, we must reduce the multiprogramming level (or install more memory while the system is running, which is not possible with current technology). That is, we have a connection between memory management and process management. These are the suspend/resume arcs we saw way back when and are shown again in the diagram on the right.

When the PFF (or another indicator) is too high, we choose a process and suspend it, thereby swapping it to disk and releasing all its frames. When the frequency gets low, we can resume one or more suspended processes. We also need a policy to decide when a suspended process should be resumed even at the cost of suspending another.

This is called medium-term scheduling. Since suspending or resuming a process can take seconds, we clearly do not perform this scheduling decision every few milliseconds as we do for short-term scheduling. A time scale of minutes would be more appropriate.

3.5.3 Page Size

Page size must be a multiple of the disk block size. Why?
Answer: When copying out a page if you have a partial disk block, you must do a read/modify/write (i.e., 2 I/Os).

Characteristics of a large page size.

Good for demand paging I/O:
We will learn later this term that the total time for performing 8 I/O operations each of size 1KB is much larger that the time for a single 8KB I/O. Hence it is better to swap in/out one big page than several small pages.
But if the page is too big you will be swapping in data that are not local and hence might well not be used.
Large internal fragmentation (1/2 page size).
Small page table (process size / page size * size of PTE).
These last two can be analyzed together by setting the derivative of the sum equal to 0. The minimum overhead occurs at a page size of
```
        sqrt(2 * process size * size of PTE)
      
```
Since the term inside the sqrt is typically megabytes, we see that modern practice of having the page size a few kilobytes is near the minimum point.
A very large page size leads to very few pages. A process will have many faults if it references more regions than the number of (large) frames that the process has been allocated.

A small page size has the opposite characteristics.

Homework: Consider a 32-bit address machine using paging with 8KB pages and 4 byte PTEs. How many bits are used for the offset and what is the size of the largest page table? Repeat the question for 128KB pages.

3.5.4 Separate Instruction and Data (I and D) Spaces

This was used when machine have very small virtual address spaces. Specifically the PDP-11, with 16-bit addresses, could address only 2₁₆ bytes or 64KB, a severe limitation. With separate I and D spaces there could be 64KB of instructions and 64KB of data.

Separate I and D are no longer needed with modern architectures having large address spaces.

3.5.5 Shared pages

Permit several processes to each have the same page loaded in the same frame. Of course this can only be done if the processes are using the same program and/or data.

Really should share segments.
Must keep reference counts or something so that, when a process terminates, pages it shares with another process are not automatically discarded.
Similarly, a reference count would make a widely shared page (correctly) look like a poor choice for a victim.
A good place to store the reference count would be in a structure pointed to by both PTEs. If stored in the PTEs themselves, we must keep somehow keep the count consistent between processes.
If you want the pages to be initially shared for reading but want each process's updates to be private, then use so called copy on write techniques.

Homework: Can a page shared between two processes be read-only for one process and read-write for the other?

3.5.6 Shared Libraries (Dynamic-Linking)

In addition to sharing individual pages, process can share entire library routines. The technique used is called dynamic linking and the objects produced are called shared libraries or dynamically-linked libraries (DLLs). (The traditional linking you did in lab1 is today often called static linking).

With dynamic linking, frequently used routines are not linked into the program. Instead, just a stub is linked.
When the routine is called (or when the process begins), the stub checks to see if the real routine has been loaded by another program).
- If it has not been loaded, load it (really page it in as needed).
- If it is already loaded, share it. The read-write data must be shared copy-on-write.
Advantages of dynamic linking.
- Saves RAM: Only one copy of a routing is in memory even when it is used concurrently by many processes. For example even a big server with hundreds of active processes will have only one copy of printf in memory. (In fact with demand paging only part of the routine will be in memory.)
- Saves disk space: Files containing executable programs no longer contain copies of the shared libraries.
- A bug fix to a dynamically linked library fixes all applications that use that library, without having to relink these applications.
Disadvantages of dynamic linking.
- New bugs in dynamically linked library infect all applications.
- Applications change even when they haven't changed.
A Technical Difficulty with dynamic linking. The shared library has different virtual addresses in each process so addresses relative to the beginning of the module cannot be used (they would need to be relocated to different addresses in the multiple copies of the module). Instead position-independent code must be used. For example, jumps within the module would use PC-relative addresses.

3.5.7 Mapped Files

The idea of memory-mapped files is to use the mechanisms in place for demand paging (and segmentation, if present) to implement I/O.

A system call is used to map a file into a portion of the address space. (No page can be part of a file and part of regular memory; the mapped file would be a complete segment if segmentation is present).

The implementation of demand paging we have presented assumes that the entire process is stored on disk. This portion of secondary storage is called the backing store for the pages. Sometimes it is called a paging disk. For memory-mapped files, the file itself is the backing store.

Once the file is mapped into memory, reads and writes become loads and stores.

3.5.8 Cleaning Policy (Paging Daemons)

Done earlier

The only point to add is now that we know replacement algorithms one can suggest an implementation. If a clock-like algorithm is used for victim selection, one can have a two handed clock with one hand (the paging daemon) staying ahead of the other (the one invoked by the need for a free frame).

The front hand simply writes out any page it hits that is dirty and thus the trailing hand is likely to see clean pages and hence is more quickly able to find a suitable victim.

Unless specifically requested, you may ignore paging daemons when answering exam questions.

3.5.9 Virtual Memory Interface

Skipped.