8.1 Demand Paging Paging as described in Chapter 7 implied whole program was in memory But does it have to be? On average – 30% of a programs memory footprint is the primary logic of program – 70% is little used error handling Therefore, prudent for memory manager not to load entire program into memory on startup. Basic idea is to load parts of the program that are not in memory on demand. This technique, referred to as demand paging results in better memory utilization.
What would be the main advantage of demand paging?
8.1.1 Hardware for demand paging 0x0023 0x0124 0x1111 0x3F04 0x0000 0x0044 0x0068 0x0 0x1 0x2 0x3 0x3E0 0x3E1 0x3E2 003E v PTBR CPU Memory
8.1.1 Hardware for demand paging If I5 page faults…handle If I2 page faults Let I 1 complete and squash I 3 -I 5 before INT. INT needs to save PC corresponding to I 2 for re-starting I 2 after servicing page fault. Note that there is no harm in squashing instructions I 3 -I 5 since they have not modified permanent state of program IFID/RREXMEMWB Instruction in Instruction out BUFFERBUFFER BUFFERBUFFER BUFFERBUFFER BUFFERBUFFER Potential page faults I1I1 I2I2 I3I3 I4I4 I5I5
8.1.2 Page fault handler 1.Find a free page frame 2.Load the faulting virtual page from the disk into the free page frame 3.Update the page table for the faulting process 4.Place the PCB of the process back in the ready queue of the scheduler
8.1.3 Data structures for Demand- paged Memory Management Free-list of page frames Frame table Disk map
8.1.3 Data structures for Demand- paged Memory Management Free-list of page frames … Pframe 52Pframe 20 Pframe 8 Free-list Pframe 200
8.1.3 Data structures for Demand- paged Memory Management Frame table free free free Pframe Note: There are different ways of implementing this. Note: There are different ways of implementing this.
8.1.3 Data structures for Demand- paged Memory Management Disk map Swap space P1P1 …..….. P2P2 PnPn disk address VPN Disk map for P1 disk address
8.1.4 Anatomy of a Page Fault Find a free page frame Pick victim page and evict Load faulting page Update page table for faulting process and frame table Restart faulting process
Eviction When evicting a page we must consider its status – Clean: the page has not been written to and thus matches its counterpart on disk – Dirty: the page has been written to and no longer matches what is on disk Clean pages may be simply evicted Dirty pages must be written back to disk
8.2 Interaction between the Process Scheduler and Memory Manager CPU Hardware CPU scheduler … PCB 1 PCB 2 ready_q Kernel Process 1 Process 2Process n ………. User level Memory Manager Timer interrupt Page fault Process dispatch … freelist Pframe PT 1 PT DM 1 DM FT (1) (2)
CPU Hardware CPU scheduler … PCB 1 PCB 2 ready_q Kernel Process 1 Process 2Process n ………. User level Memory Manager Timer interrupt Page fault Process dispatch … freelist Pframe PT 1 PT DM 1 DM FT (1) (2) CPU scheduler dispatches process, it runs until one of following happens 1.HW timer interrupts CPU causing upcall (1) to CPU scheduler that may result in a process switch. CPU scheduler takes appropriate action to schedule next process on CPU. 2.Process incurs a page fault resulting in an upcall (2) to memory manager that results in page fault handling 3.Process makes system call resulting in another subsystem (not shown) getting an upcall
8.3 Page Replacement Policies How to pick victim page to evict from physical memory when page fault & free-list is empty. For a given string of page references, policy should result in least number of page faults. – This attribute ensures that the amount of time spent in OS dealing with page faults is minimized. Ideally, once a particular page has been brought into physical memory, policy should not incur a page fault for same page again. – This attribute ensures that page fault handler attempts to respect reference pattern of the user programs.
8.3 Page Replacement Policies May use – Local victim selection Simple Don't need frame table Poor memory utilization – Global victim selection Better memory utilization The norm Ideally there are no page faults and memory manager never runs. Goal is to minimize (or eliminate) page faults
8.3.1 Belady’s Min In 1966 Laszlo Belady proposed an optimal page replacement algorithm requiring to know in advance the page replacement string Obviously this is impossible But the performance level of Belady's Min may be used as a reference standard to compare to other policies performance
8.3.2 First In First Out (FIFO) Affix a timestamp when a page is brought in to physical memory If a page has to be replaced, choose the longest resident page as the victim No special hardware needed Queue length is number of physical frames Circular queue ….. Head Tail free Full
FIFO Maintain queue. As page is read in enqueue. Use head of queue as frame to replace Sample 1,2,3,4,1,2,5,1,2,3,4,5
FIFO Belady’s Anomaly Time
8.3.3 Least Recently Used (LRU) LRU policy makes assumption that if a page has not been referenced in a long time there is a good chance it will not be referenced in the future as well. Thus, victim page in LRU policy is page that has not been used for longest time. Push down stack ….. Top Bottom free
8.3.3 Least Recently Used (LRU) TIME Physical Frames Push Down Stack
8.3.3 Least Recently Used (LRU) LRU is appealing but actually not feasible – Stack has as many entries as number of physical frames. For a physical memory of 64 MB & an 8 KB pagesize, size of stack: 8 KB. Too big in datapath! – On every access, hardware has to modify stack to place current reference on top of stack. Too slow. LRU may be bad choice in certain situations – e.g. Access N+1 pages in a processor with N frames available
Approximate LRU: A Small Hardware Stack Add a hardware stack with ~16 entries Push references onto stack – If they are already in stack bring to top – Bottom reference falls out of stack When free frame needed randomly select one not in stack Shown to be successful in some applications Probably not fast enough for high speed pipelined processor
Approximate LRU: Reference bit per page frame Associate a bit with each frame. – hardware sets on reference – software reads and clears Have an n-bit counter register for each frame Periodically (daemon) right shifts all counters and puts reference bit into high order bit Highest value counters are recently used frames; lowest value counters are LRU frames
8.3.4 Second chance page replacement algorithm Initially, OS clears reference bits of all frames. As program executes, hardware sets reference bits for pages referenced by program. If a page has to be replaced, memory manager chooses replacement candidate in FIFO manner. If chosen victim’s reference bit is set, then manager clears reference bit and this page is moved to end of FIFO queue. The victim is first candidate in FIFO order whose reference bit is not set.
8.3.5 Review of page replacement algorithms PAGE REPLACEMENT ALGORITHM HARDWARE ASSIST NEEDED COMMENTS FIFONone Could lead to anomalous behavior Belady’s MINOracle Provably optimal performance; not realizable in hardware; useful as a standard for performance comparison True LRUPush down stack Expected performance close to optimal; infeasible for hardware implementation due to space and time complexity; worst- case performance may be similar or even worse compared to FIFO Approximate LRU #1A small hardware stack Expected performance close to optimal; worst-case performance may be similar or even worse compared to FIFO Approximate LRU #2Reference bit per page Expected performance close to optimal; moderate hardware complexity; worst-case performance may be similar or even worse compared to FIFO Second Chance Replacement Reference bit per page Expected performance better than FIFO; memory manager implementation simplified compared to LRU schemes
8.3.6 Optimizing Memory Management Beyond basic techniques presented additiona optimizations are possible These optimizations are on top of the already presented techniques
8.3.7 Pool of free page frames Instead of waiting for free page count to = 0 Periodically run daemon to evict pages keeping a pool of n free pages
Overlapping I/O with Processing Upon eviction we add the evicted frame to the free list If the frame was dirty it is scheduled for write back When a frame is needed only clean frames are selected skipping over dirty frames still awaiting write back
Reverse Mapping to Page Tables When daemon runs to maintain free list at a certain level it will take pages from processes that might turn around and page fault on those pages If we maintain additional info in free list we can know this and give page back to process (since the data is still intact) freelist Pframe 52 Dirty Pframe 22 Clean Pframe 200 Clean ….….
8.3.8 Thrashing Suppose many processes are in memory but CPU utilization is low. What could cause this? 1.Too many I/O bound processes? 2.Too many CPU bound processes? Should we add more processes into memory?
8.3.8 Thrashing If some processes don't really have enough pages to support their current configuration they will constantly be page faulting and trying to grab frames from other processes This may lead those processes to also start page faulting and grabbing frames Does this sound like the ideal place to introduce more processes into memory?
8.3.8 Thrashing Controlling thrashing starts with understanding temporal locality Temporal locality is the tendency for the same memory location to be accessed over a short period of time During a given time period t certain pages will be accessed others will not If during t the pages that need to be accessed are in memory then no page faults will occur
Frames Pages Memory Reference
8.3.9 Working set Working set is the set of pages that defines the locus of activity of a program The working set size (WSS) denotes the number of distinct pages touched by a process in a window of time. The total memory pressure (TMP) exerted on the system is the summation of the WSS of all the processes currently competing for resources.
Controlling thrashing 1.If TMP > Physical Memory – Decrease the degree of multiprogramming. – Else: Increase # physical frames per process Page fault rate High water mark Low water mark 2.Monitor Page fault rate if pfr>High Decrease progs if pfr
8.4 Other considerations Prepaging after swapping out – Bring entire WS back when swapping back in Memory manager and I/O system must work together – Suppose I/O system is writing data into page that is being swapped out by memory manager! – Solution: Pining – What about pages where memory manager lives? – They are pined
8.5 Speeding up Address Translation We will do whatever we can to reduce page fault rate – Context switch time < 100 instructions – Page fault time without disk I/O < 100 instructions – Disk I/O to handle page fault ~1,000,000 instructions Nevertheless there remains a big penalty. Each processor memory reference actually requires two memory accesses! – One to page table – One to actual location desired
8.5 Speeding up Address Translation Consider spatial locality which is the tendency to access nearby locations over a short period of time If we access address 0x how likely is it that we will access 0x C? What frame will we access in each case? Recall – 0x – 0x C
8.5.1 Address Translation with TLB Solution: Install a hardware device which stores recently accessed page table info Divided into two sections one for user entries, the other for kernel entries – On context switch user portion may be flushed quickly with a special kernel-mode instruction
CPU PageOffset PageFrame PageFrame PageFrame PageFrame PageFrame PageFrame PageTable Physical Memory FrameOffset TLB TLB Miss
CPU PageOffset PageFrame PageFrame PageFrame PageFrame PageFrame PageFrame Page Table Physical Memory FrameOffset TLB TLB Hit
8.6 Advanced topics in memory management How big is a page table? Assume a 32 bit address and a page size of 4kB Assume each page table entry is 32 bits long And each process has a page table! How can we make this work?
8.6.1 Multi-level Page Tables To deal with excessively large page tables we can page the page table Page 1Page 2Offset Page of Page Table Physical Memory Outer Page Table PTBR
8.6.2 Sophisticated use of the page table entry Page tables may contain more than just a physical frame number They may also contain mode information – RW: Read Write – RO: Read Only – CW: Copy on Write Useful for items such as static-read-only areas of memory and process forking
8.6.3 Inverted page tables Virtual memory is usually much larger than physical memory Some architectures (e.g. IBM Power processors) use an inverted page table, essentially a frame table. The inverted page table alleviates the need for a per-process page table. Size of table is equal to size of physical memory (in frames) rather than virtual memory. Unfortunately, inverted page tables complicate logical to physical address translation done by hardware. – hardware handles address translations through TLB mechanism. – On TLB miss, hardware hands over control (through a trap) to OS to resolve translation in software. OS is responsible for updating TLB as well. – Architecture usually provides special instructions for reading, writing, and purging TLB entries in privileged mode.
8.7 Summary Topics Demand paging basics including hardware support, and data structures in OS for demand-paging Interaction between CPU scheduler and memory manager in dealing with page faults Page replacement policies including FIFO, LRU, and second chance replacement Techniques for reducing penalty for page faults including keeping a pool of page frames ready for allocation on page faults, performing any necessary writes of replaced pages to disk lazily, and reverse mapping replaced page frames to the displaced pages Thrashing and the use of working set of a process for controlling thrashing Translation look-aside buffer for speeding up address translation to keep the pipelined processor humming along Advanced topics in memory management including multi-level page tables, and inverted page tables