Operating System Three easy pieces Remzi H ArpaciDusseau

We dreamed a dream § I dreamed a dream § Every address space of

Swap Space § Be reserved on the storage that larger, slower than main memory

Swap Space § 4 pages in Memory Physical Memory Swap Space PFN 0 PFN

Swap Space § 4 pages in Memory Proc 1 needs second page Physical Memory

Swap Space § 4 pages in Memory Swap out vpn 1 of proc 0

Swap Space § 4 pages in Memory Load VPN 1 of Proc 1 Physical

The Present Bit § A bit in PTE(Page Table Entry) § Represents whether the

The Page Fault § Page Fault Handler in OS will service the case §

What If Memory Is Full? § What if there are no page to be

Page Fault Control Flow VPN = (Virtual. Address & VPN_MASK) >> SHIFT (Success, Tlb.

Page Fault Control Flow if (Success == True) // TLB Hit. . . else

Page Fault Control Flow PFN = Find. Free. Physical. Page() If (PFN == -1)

When Replacements Really Occur § OS waits until memory is entirely full, and then

Summary § Introduced the notion of accessing more memory than is physically present within

Slides: 44

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Operating System : Three easy pieces Remzi H. Arpaci-Dusseau Andrea C. Arpaci-Dusseau Beyond Physical Memory: Mechanisms Seong. Jae Park (sjpark@dcslab. snu. ac. kr) School of Computer Science and Engineering Seoul National University

We dreamed a dream § I dreamed a dream § Every address space of every running process fits into physical memory § The life have killed the dream § Physical memory is smaller than address space, usually § For 32 bits system, 4 Gi. B address space § I have a dream! § Wish to support many concurrently-running large address spaces § TL; DR: Place to stash away portions of address spaces that currently aren’t in great demand § When, how the pages be stashed and be inserted to memory again? Operating System : Three easy pieces 2

Swap Space § Be reserved on the storage that larger, slower than main memory § Usually Hard Disk Drive § Space to stash away portions of address spaces § Swap pages out of memory to swap space, and swap pages into memory from swap space when necessary Physical Memory Swap Space PFN 0 PFN 1 PFN 2 PFN 3 Proc 0 [VPN 0] Proc 1 [VPN 2] Proc 1 [VPN 3] Proc 2 [VPN 0] Block 0 Block 1 Block 2 Block 3 Block 4 Proc 0 [VPN 1] Proc 0 [VPN 2] Proc 1 [VPN 0] Proc 1 [VPN 1] Proc 3 [VPN 0] Operating System : Three easy pieces 4

Swap Space § 4 pages in Memory Physical Memory Swap Space PFN 0 PFN 1 PFN 2 PFN 3 Proc 0 [VPN 0] Proc 0 [VPN 1] Proc 0 [VPN 2] Free Block 0 Block 1 Block 2 Block 3 Block 4 Free Free Operating System : Three easy pieces 5

Swap Space § 4 pages in Memory Physical Memory Swap Space PFN 0 PFN 1 PFN 2 PFN 3 Proc 0 [VPN 0] Proc 0 [VPN 1] Proc 0 [VPN 2] Proc 1 [VPN 0] Block 0 Block 1 Block 2 Block 3 Block 4 Free Free Operating System : Three easy pieces 6

Swap Space § 4 pages in Memory Proc 1 needs second page Physical Memory Swap Space PFN 0 PFN 1 PFN 2 PFN 3 Proc 0 [VPN 0] Proc 0 [VPN 1] Proc 0 [VPN 2] Proc 1 [VPN 0] Block 0 Block 1 Block 2 Block 3 Block 4 Free Free Operating System : Three easy pieces 7

Swap Space § 4 pages in Memory Swap out vpn 1 of proc 0 Physical Memory Swap Space PFN 0 PFN 1 PFN 2 PFN 3 Proc 0 [VPN 0] Free Proc 0 [VPN 2] Proc 1 [VPN 0] Block 0 Block 1 Block 2 Block 3 Block 4 Proc 0 [VPN 1] Free Operating System : Three easy pieces 8

Swap Space § 4 pages in Memory Load VPN 1 of Proc 1 Physical Memory Swap Space PFN 0 PFN 1 PFN 2 PFN 3 Proc 0 [VPN 0] Proc 1 [VPN 1] Proc 0 [VPN 2] Proc 1 [VPN 0] Block 0 Block 1 Block 2 Block 3 Block 4 Proc 0 [VPN 1] Free Operating System : Three easy pieces 9

The Present Bit § A bit in PTE(Page Table Entry) § Represents whether the page is in memory or not § If not, page fault will be incurred § The fault will be handled by page fault handler in OS Page table of Proc 0 Physical Memory Swap Space PFN 0 PFN 1 PFN 2 PFN 3 Proc 0 [VPN 0] Proc 1 [VPN 2] Proc 1 [VPN 3] Proc 2 [VPN 0] Block 0 Block 1 Block 2 Block 3 Block 4 Proc 0 [VPN 1] Proc 0 [VPN 2] Proc 1 [VPN 0] Proc 1 [VPN 1] Proc 3 [VPN 0] Operating System : Three easy pieces VPN Present 0 1 1 0 2 0 10

The Page Fault § Page Fault Handler in OS will service the case § Swap the page into memory from swap space § How will the OS know where to find the page? § Bits in PTE, usually PFN(Page Frame Number) Proc 0 VPN 1. . Oops! Page table of Proc 0 Physical Memory Swap Space PFN 0 PFN 1 PFN 2 PFN 3 Proc 0 [VPN 0] Proc 1 [VPN 2] Proc 1 [VPN 3] Proc 2 [VPN 0] Block 0 Block 1 Block 2 Block 3 Block 4 Proc 0 [VPN 1] Proc 0 [VPN 2] Proc 1 [VPN 0] Proc 1 [VPN 1] Proc 3 [VPN 0] Operating System : Three easy pieces VPN Present 0 1 1 0 2 0 11

What If Memory Is Full? § What if there are no page to be swapped in? § Page out one or more pages to make room for the new page § Follows page-replacement policy § Good page-replacement policy is important • Wrong decision cause a program to run at disk-like speeds(10, 000 x – 100, 000 x slower) Physical Memory Swap Space PFN 0 PFN 1 PFN 2 PFN 3 Proc 0 [VPN 0] Proc 0 [VPN 1] Proc 0 [VPN 2] Proc 1 [VPN 0] Proc 1 needs second page Block 0 Block 1 Block 2 Block 3 Block 4 Free Free Operating System : Three easy pieces 12

Page Fault Control Flow VPN = (Virtual. Address & VPN_MASK) >> SHIFT (Success, Tlb. Entry) = TLB_Lookup(VPN) if (Success == True) // TLB Hit if (Can. Access(Tlb. Entry. Protect. Bits) == True) Offset = Virtual. Address & OFFSET_MASK Phys. Addr = (Tlb. Entry. PFN << SHIFT) | Offset Register = Access. Memory(Phys. Addr) else Rasie. Exception(PROTECTION_FAULT) else. . . Operating System : Three easy pieces 13

Page Fault Control Flow if (Success == True) // TLB Hit. . . else PTEAddr = PTBR + (VPN * sizeof(PTE)) PTE = Access. Memory(PTEAddr) if (PTE. Valid == False) Raise. Exception(SEGMENTATION_FAULT) else if (Can. Access(PTE. Protect. Bits) == False) Raise. Exception(PROTECTION_FAULT) else if (PTE. Present == True) TLB_Insert(VPN, PTE. PFN, PTE. Protect. Bits) Retry. Instruction() else if (PTE. Present == False) Raise. Exception(PAGE_FAULT) Operating System : Three easy pieces 23

Page Fault Control Flow PFN = Find. Free. Physical. Page() If (PFN == -1) PFN = Evict. Page() Disk. Read(PTE. Disk. Addr, pfn) PTE. present = True PTE. PFN = PFN Retry. Instruction() PFN 0 PFN 1 PFN 2 PFN 3 Proc 0 [VPN 0] Proc 0 [VPN 1] Proc 0 [VPN 2] Proc 1 [VPN 0] Block 0 Block 1 Block 2 Block 3 Block 4 Proc 1 [VPN 1] Free Physical Memory Swap Space Operating System : Three easy pieces 38

Page Fault Control Flow PFN = Find. Free. Physical. Page() If (PFN == -1) PFN = Evict. Page() Disk. Read(PTE. Disk. Addr, pfn) PTE. present = True PTE. PFN = PFN Retry. Instruction() PFN 0 PFN 1 PFN 2 PFN 3 Proc 0 [VPN 0] Free Proc 0 [VPN 2] Proc 1 [VPN 0] Block 0 Block 1 Block 2 Block 3 Block 4 Proc 1 [VPN 1] Proc 0 [VPN 1] Free Physical Memory Swap Space Operating System : Three easy pieces 39

Page Fault Control Flow PFN = Find. Free. Physical. Page() If (PFN == -1) PFN = Evict. Page() Disk. Read(PTE. Disk. Addr, pfn) PTE. present = True PTE. PFN = PFN Retry. Instruction() Physical Memory Swap Space Will transit to wait status until the IO completes Proc 0 [VPN 0] Proc 1 [VPN 1] Proc 0 [VPN 2] Proc 1 [VPN 0] Block 0 Block 1 Block 2 Block 3 Block 4 Free Proc 0 [VPN 1] Free Operating System : Three easy pieces 40

When Replacements Really Occur § OS waits until memory is entirely full, and then evicts a page to make room for some other page § Unrealistic § Keep high watermark and low watermark § Start swap daemon if free memory < low watermark § Stop swap daemon if free memory > high watermark Operating System : Three easy pieces 43

Summary § Introduced the notion of accessing more memory than is physically present within a system § Requires more complexity in page-table structures § Present bit in PTE represents whether the page is in memory or not § If present bit == 0, page fault occur to give control to page-fault handler in OS § Actions all take place transparently to the process § Process is just accessing its own private, contiguous virtual memory § In the worst case, a single instruction could consume few milliseconds to complete Operating System : Three easy pieces 44