1Adapted from UC Berkeley CS252 S01

Lecture 19: Virtual Memory

Virtual Memory concept, Virtual-physical translation, page table, TLB, Alpha 21264 memory hierarchy

Virtual MemoryVirtual memory (VM) allows programs to have the illusion of a very large memory that is not limited by physical memory size

Make main memory (DRAM) acts like a cache for secondary storage (magnetic disk)Otherwise, application programmers have to move data in/out main memoryThat’s how virtual memory was first proposed

Virtual memory also provides the following functionsAllowing multiple processes share the physical memory in multiprogramming environmentProviding protection for processes (compare Intel 8086: without VM applications can overwrite OS kernel)Facilitating program relocation in physical memory space

3

VM Example

4

Virtual Memory and CacheVM address translation a provides a mapping from the virtual address of the processor to the physical address in main memory and secondary storage.

Cache terms vs. VM termsCache block => pageCache Miss => page fault

Tasks of hardware and OSTLB does fast address translations OS handles less frequently events:

page faultTLB miss (when software approach is used)

Virtual Memory and Cache

Parameter L1 Cache Main Memory

Block (page) size 16-128 bytes 4KB – 64KB

Hit time 1-3 cycles 50-150 cycles

Miss Penalty 8-300 cycles 1M to 10M cycles

Miss rate 0.1-10% 0.00001-0.001%

Address mapping 25-45 bits => 13-21 bits

32-64 bits => 25-45 bits

4 Qs for Virtual MemoryQ1: Where can a block be placed in the upper level?

Miss penalty for virtual memory is very high => Full associativity is desirable (so allow blocks to be placed anywhere in the memory)Have software determine the location while accessing disk (10M cycles enough to do sophisticated replacement)

Q2: How is a block found if it is in the upper level?

Address divided into page number and page offset Page table and translation buffer used for address translationQ: why fully associativity does not affect hit time?

7

4 Qs for Virtual MemoryQ3: Which block should be replaced on a miss?

Want to reduce miss rate & can handle in softwareLeast Recently Used typically usedA typical approximation of LRU

Hardware set reference bitsOS record reference bits and clear them periodicallyOS selects a page among least-recently referenced for replacement

Q4: What happens on a write?Writing to disk is very expensiveUse a write-back strategy

8

Virtual and Physical AddressesA virtual address consists of a virtual page number and a page offset. The virtual page number gets translated to a physical page number.The page offset is not changed

Virtual Page Number Page offset

Physical Page Number Page offset

Translation

Virtual Address

Physical Address

36 bits

33 bits

12 bits

12 bits

9

Address Translation Via Page Table

Assume the access hits in main memory

Address Translation with Page TablesA page table translates a virtual page number into a physical page numberA page table register indicates the start of the page table.The virtual page number is used as an index into the page table that contains

The physical page numberA valid bit that indicates if the page is present in main memoryA dirty bit to indicate if the page has been writtenProtection information about the page (read only, read/write, etc.)

Since page tables contain a mapping for every virtual page, no tags are required (how to compare it with cache?)

Page table access is slow; we will see the solution

11

Page Table Diagram

12

Accessing Main Memory or DiskValit bit being zero means the page is not in main memoryThen a page fault occurs, and the missing page is read in from disk.

13

How Large Is Page Table?Suppose

48-bit virtual address41-bit physical address8 KB pages => 13 bit page offsetEach page table entry is 8 bytes

How large is the page table?Virtual page number = 48 - 13 = 25 bytesNumber of entries = number of pages = 225 = 32MTotal size = number of entries x bytes/entry

= 32M x 8B = 256 MbytesEach process needs its own page table

Page tables have to be very large, thus must be stored in main page or even paged, resulting in slow accessWe need techniques to reduce page table size

14

TLB: Improving Page Table AccessCannot afford accessing page table for every access include cache hits (then cache itself makes no sense)Again, use cache to speed up accesses to page table! (cache for cache?)TLB is translation lookaside buffer storing frequently accessed page table entryA TLB entry is like a cache entry

Tag holds portions of virtual address Data portion holds physical page number, protection field, valid bit, use bit, and dirty bit (like in page table entry)Usually fully associative or highly set associativeUsually 64 or 128 entries

Access page table only for TLB misses

15

TLB CharacteristicsThe following are characteristics of TLBs

TLB size : 32 to 4,096 entriesBlock size : 1 or 2 page table entries (4 or 8 bytes each)Hit time: 0.5 to 1 clock cycleMiss penalty: 10 to 30 clock cycles (go to page table)Miss rate: 0.01% to 0.1%Associative : Fully associative or set associativeWrite policy : Write back (replace infrequently)

16

Alpha 21264 Data TLB128 entries, fully associativeASN (like PID) to avoid flushingAlso check protection

17

Determine Page SizeLarger Size Comments

Page table size Inversely proportionalFast L1 cache hit L1 cache can be largerI/O utilization Longer burst

transferTLB hit rate Increasing TLB coverageStorage efficiency Reducing fragmentationI/O efficiency Unnecessary data

transferProcess start-up Small processes are

popular

Most commonly used size: 4KB or 8KBHardware may support a range of page sizesOS selects the best one(s) for its purpose

18

Alpha 21264 TLB Access

Virtual indexedPhysically tagged

Physically indexedPhysically tagged

19

Alpha 21264 Virtual MemoryCombining segmentation and paging

Segmentation: variable-size memory space range, usually defined by a base register and a limit fieldSegmentation assign meanings to address spaces, and reduce address space that needs paging (reducing page table size)Paging is used on the address space of each segment

Three segments in Alphakseg: reserved for OS kernel, not VM managementseg0: virtual address accessible to user processseg1: virtual address accessible to OS kernel

20

Two Viewpoints of Virtual MemoryApplication programs

Sees a large, flat memory spaceAssumes fast access to every placeHardware/OS hide the complexity

OS KernelManages multiple process spacesReserves direct accesses to some portions of physical memoryMay access physical memory, its own virtual memory, and virtual memory of the current processHardware facilitates fast VM accesses, and OS manages slow, less frequent events

21

Alpha 21264 Page Table10-bit

1024 8B PTEs

13-bit

13-bit28-bit

Page table access on TLB miss managed bysoftware

22

Memory Protection Memory protection: preventing unauthorized accesses to process and kernel memoryMemory protection implementation:

User programs can only access through virtual memoryPTE entry contains protection bits to allow shared but protected accesses

Protection fields in AlphaValid, user read enable, kernel read enable, user write enable, and kernel write enable

23

Memory Hierarchy Example:Alpha 21264 in AlphaServer ES40

L1 instruction cache: 2-way, 64KB, 64-byte block, Virtually indexed and tagged

Use way prediction and line prediction to allow instruction fetching

Inst prefetcher: store four prefetched instructions, accessed before L2 cacheL1 data cache: 2-way, 64KB, 64-byte block, Virtually indexed, physically tagged, write-throughVictim buffer: 8-entry, checked before L2 accessL2 unified cache: 1-way 1MB to 16MB, off-chip, write-back;

Allow critical-word transfer to L1 cache, transfers 16B per 2.25ns

TLB: 128-entry fully associative for inst and data (each)ES40: L1 miss penalty 22ns, L2 130 ns; up to 32GB memory; 256-bit memory buses (64-bit into processor)Read 5.13 for more details