CS 3733 Operating Systems Notes: Virtual Memory Examples
Real Page Tables
Real page tables often contain information in addition to a valid bit and
a frame number. Some typical bits include:
- dirty: indicates that the page has changed since it was read from disk
- read-only: an interrupt is generated if a write is performed
- execute: if not set, code cannot be executed from this page
- cache-disabled: if set, do not put this in the cache
- guard: any access to this page generates an interrupt
Real page tables often have several levels:
Address Translation in Win 2000
Address Translation in the Alpha AXP Model 21064
64-bit virtual address (43 bits used)
64-bit page table entries
8K page size
2 10 10 10 13
|seg| unused | Level 1 | Level 2 | Level 3 | Offset |
seg=00 and seg=01: seg0 = text and heap
seg=10: kseg - kernel, no memory management
seg=11: seg - stack
- MMU holds physical address of L1 page table.
- L1 page table is locked in memory.
- L1 page table entries contain virtual address of L2 page table.
- L2 page table entries contain virtual address of L3 page table.
- L3 page table entries contain:
- physical frame number (32 bits).
- 5 protection fields:
user read enable
kernel read enable
user write enable
kernel write enable
- A page fault can occur when accessing L2 and L3 page tables.
- Each page table is 1K entries (10 bits), each entry is 8 bytes
- Each page table fits in one page of memory.
- TLB: 32 entries for data, 8 entries for instructions
- hit time: 1 cycle
- miss penalty: about 20 cycles
- physical addresses limited to 34 bits
Traditional Unix Page Replacement
- Called Global Clock Least Recently Used
- Like Second Chance, but does not require a reference bit
- Non-kernel memory is swept in a circular fashion by a software clock hand
- When the clock hand reaches a given frame, the page table entry is located
- This requires an inverted page table
- An inverted page table is an array with one entry per frame containing
a pid and a page number.
- If frame is free, do nothing
- If frame is being used by a pending I/O instruction, do nothing
- If frame is valid in some page table, mark it invalid but
keep the association with the page.
- If frame is associated with a page but marked invalid in page table,
free the frame (no longer associated with the page).
- The clock is controlled by the pagedaemon, process 2.
- The pagedaemon spends most of its time sleeping.
- It wakes up when the number of free frames is below lotsfree.
- If this needs to be done too often, the swapper activates.
Page Replacement in Solaris
- When a page fault occurs, a frame from the free frame list is assigned.
- Must keep a large number of free frames
- If number of free pages falls below lotsfree the pageout
- lotsfree is typically about 1/64 of total physical memory.
- pageout is similar to second chance.
- The algorithm is known as the clock or 2-handed clock algorithm.
- It uses both a hardware reference bit and a dirty bit,
both kept in the MMU.
- The first hand scans all frames setting the reference bit to 0.
- A second hand runs behind the first one. If it finds a reference bit
is 0, it frees the frame. If the dirty bit is set, the corresponding
page must be written back to memory.
- The distance between the hands is called the handspread and the rate of
scanning is called the scanrate (in frames per second). They determine
how often a page needs to be referenced to be kept in memory.
- The scanrate can range from 100 frames per second to 8192 frames per
- If pageout cannot keep enough frames free, some processes are
removed from memory (swapping).
Page Replacement in Win 2000
- Maintain a substantial number of free pages.
- This means that a page fault can be satisfied by a single disk access, a read.
- The major part of the algorithm is how pages are taken away from
processes and given to the free list.
- Each process has a working set described by two parameters, min and max.
- Here the "working set" means the number of frames allocated to the process.
- These are not hard limits and a process may have fewer frames than min
or more than max.
- Each process starts with the same min and max, but these may change
- min is typically 20-50 and max is typically 45-345, depending on total ram.
- If a page fault occurs and the working set is smaller than the max,
a page is added.
- If it is larger than the max, a page is removed, but stays in memory.
- Once a second, the balance set manager checks if there are enough
- If not it starts the working set manager to recover more frames,
starting with large processes that have been idle for a while.
- Each page is examined.
- Each page has a counter associated with it:
if reference bit is set, counter is cleared
if reference bit is clear, counter in incremented
pages with highest counters are removed
- Reference bit is part of the CPU, so this is only used for uniprocessors.
- On multiprocessors, FIFO is used.
- There are 4 free lists:
standby: contain clean pages that can be restored to processes
modified: contain dirty pages that can be restored to processes
free: contain frames not associated with a process
zeroed: these are free and contain 0's
- There is also a list of physically bad RAM frames
- Standby and modified lists allow for fast (soft) page faults.