Chapter 21Swapping: Mechanisms

Chien-Chung ShenCIS, UD

cshen@cis.udel.edu

Go Beyond Physical Memory

• How to support many concurrently-running large address spaces ?– OS needs a place to stash away portions of address

spaces that currently aren’t in great demand (where ?)• “swap space” on hard disk drive

– how can OS make use of a larger, slower device (disk) to transparently provide the illusion of a large virtual address space

• virtual memory (with swap space on disk)

– why support a single large address space for process ?• convenience and ease of use

Swap Space

• Reserved space on disk for moving pages back and forth – remember disk address of a given

page

– Code page(s) of a.out are initially on disk

TLB Algorithm (Review)VPN = (VirtualAddress & VPN_MASK) >> SHIFT (Success, TlbEntry) = TLB_Lookup(VPN)if (Success == True) // TLB Hit if (CanAccess(TlbEntry.ProtectBits) == True) Offset = VirtualAddress & OFFSET_MASK PhysAddr = (TlbEntry.PFN << SHIFT) | Offset AccessMemory(PhysAddr) else RaiseException(PROTECTION_FAULT) else // TLB Miss PTEAddr = PTBR + (VPN * sizeof(PTE)) PTE = AccessMemory(PTEAddr) if (PTE.Valid == False) RaiseException(SEGMENTATION_FAULT) else if (CanAccess(PTE.ProtectBits) == False) RaiseException(PROTECTION_FAULT) else TLB_Insert(VPN, PTE.PFN, PTE.ProtectBits) RetryInstruction()

Present Bit and Page Fault

• When hardware looks in PTE, it may find page is not present in physical memory – present bit == 0– page fault

• OS page-fault handler (for both hardware-managed and software-managed TLBs)

• why not hardware handle page fault ?– disk is too slow and too much details to handle

• where to find the desired page ?– page table (PFN or disk address)

When Memory Is Full

• OS pages out one or more pages to make room for new page(s) OS is about to page in – page-replacement policy – disk-like speed vs. memory-like speed

(10,000 or 100,000 times slower)

Page Fault Control FlowVPN = (VirtualAddress & VPN_MASK) >> SHIFT (Success, TlbEntry) = TLB_Lookup(VPN)if (Success == True) // TLB Hit if (CanAccess(TlbEntry.ProtectBits) == True) Offset = VirtualAddress & OFFSET_MASK PhysAddr = (TlbEntry.PFN << SHIFT) | Offset Register = AccessMemory(PhysAddr) else RaiseException(PROTECTION_FAULT) Else // TLB Miss PTEAddr = PTBR + (VPN * sizeof(PTE)) PTE = AccessMemory(PTEAddr) if (PTE.Valid == False) RaiseException(SEGMENTATION_FAULT) else if (CanAccess(PTE.ProtectBits) == False) RaiseException(PROTECTION_FAULT) else if (PTE.Present == True) // assuming hardware-managed TLB TLB_Insert(VPN, PTE.PFN, PTE.ProtectBits) RetryInstruction() else RaiseException(PAGE_FAULT)

When Replacement Occurs

• OS keeps a small amount of memory free by having high watermark (HW) and low watermark (LW) to help decide when to start evicting pages from memory

• When OS notices that there are fewer than LW pages available, a background thread (swap daemon or page daemon) that is responsible for freeing memory runs. The thread evicts pages until there are HW pages available

• Cluster or group a number of pages and write them out at once to the swap space, thus increasing the efficiency of the disk