Lecture 7 Memory Management

Virtual Memory Approaches Time Sharing: one process uses RAM at a time Static Relocation: statically rewrite code before run Base: add a base to virtual address to get physical Base+Bounds: also check physical is in range Segmentation: many base+bounds pairs Paging: divide mem into small, fix-sized page Too slow – TLB Too big – smaller tables

1 VPN = (VirtualAddress & VPN_MASK) >> SHIFT 2 (Success, TlbEntry) = TLB_Lookup(VPN) 3 if (Success == True) // TLB Hit 4 if (CanAccess(TlbEntry.ProtectBits) == True) 5 Offset = VirtualAddress & OFFSET_MASK 6 PhysAddr = (TlbEntry.PFN << SHIFT) | Offset 7 AccessMemory(PhysAddr) 8 else 9 RaiseException(PROTECTION_FAULT) 10 else // TLB Miss 11 PTEAddr = PTBR + (VPN * sizeof(PTE)) 12 PTE = AccessMemory(PTEAddr) 13 if (PTE.Valid == False) 14 RaiseException(SEGMENTATION_FAULT) 15 else if (CanAccess(PTE.ProtectBits) == False) 16 RaiseException(PROTECTION_FAULT) 17 else 18 TLB_Insert(VPN, PTE.PFN, PTE.ProtectBits) 19 RetryInstruction()

1 VPN = (VirtualAddress & VPN_MASK) >> SHIFT 2 (Success, TlbEntry) = TLB_Lookup(VPN) 3 if (Success == True) // TLB Hit 4 if (CanAccess(TlbEntry.ProtectBits) == True) 5 Offset = VirtualAddress & OFFSET_MASK 6 PhysAddr = (TlbEntry.PFN << SHIFT) | Offset 7 AccessMemory(PhysAddr) 8 else 9 RaiseException(PROTECTION_FAULT) 10 else // TLB Miss 11 RaiseException(TLB_MISS)

Context Switches What happens if a process uses the cached TLB entries from another process? Solutions? flush TLB on each switch remember which entries are for each process Address Space Identifier

Tag each TLB entry with an 8-bit ASID How many ASIDs to we get? What if there are more PIDs than ASIDs? 3 P1 (ASID 11) P2 (ASID 12) validVPNPFNASID ? 114? 103?

How Many Physical Accesses Assume 256-byte pages, 16-bit addresses. Assume ASID of current process is 211 0xAA10: movl 0x1111, %edi 0xBB13: addl $0x3, %edi 0x0519: movl %edi, 0xFF10 validVPNPFNASIDProt 10xBB0x91211? 10xFF0x23211? 10x050x91112? 00x050x12211?

PTE Size Phys addr space contains 1024 pages. 7 bits needed for extras (prot, valid, etc.) Phys addrs 16-bits, pages are 32 bytes, 8 bits needed for extras Phys addr space is 4 GB, pages are 4 KB, 6 bits needed for extras

Page Table Size (a) PTE’s are 3 bytes, and there are 32 possible virtual page numbers (b) PTE’s are 3 bytes, virtual addrs are 24 bits, and pages are 16 bytes (c) PTE’s are 4 bytes, virtual addrs are 32 bits, and pages are 4 KB (d) PTE’s are 4 bytes, virtual addrs are 64 bits, and pages are 4 KB (e) assume each PTE is 10 bits. We cut the size of pages in half, keeping everything else fixed, including size of virt/phys addresses. By what factor does the PT increase in size?

Page Size (a) Goal PT size is 512 bytes. PTE’s are 4 bytes. Virtual addrs are 16 bits (b) Goal PT size is 1 KB. PTE’s are 4 bytes. Virtual addrs are 16 bits (c) Goal PT size is 4 KB. PTE’s are 4 bytes. Virtual addrs are 32 bits

Now let’s solve the too big problem

Change Sizes Make virtual address space smaller Make pages bigger Why are 4 MB pages bad? Internal fragmentation.

Abandon Simple Linear Page Tables Use more complex PTs, instead of just a big array Look at the problem more closely..

Many invalid PT entries There is a big “hole” in our page table Invalid entries are clustered. How did we fix holes in virtual address space? Segmentation Paging

Segmentation/Paging Hybrid Idea: use different page tables for heap, stack, etc Each PT can have a different size Each PT has a base/bounds (where?) Virtual address Before: VPN(PT_Index) + OFFSET Now: SEG + PT_Index+ OFFSET

Segment 00: code Segment 01: heap (a) 0x12FFF => (b) 0x10FFF => (c) 0x01ABC => (d) 0x11111 => Stack? External fragmentation? PFNvalidProt 0x101r-x 0x151r-x 0x121r-x … PFNvalidProt 0x221rw- 0x021rw- 0x041rw- … 18-bit addresses. 2-bit segment index 4-bit VPN

Multi-Level Page Tables Idea: break PT itself into pages A page directory refers to pieces only have pieces with >0 valid entries Used by x86 Virtual address Basic paging: VPN(PT_Index) + OFFSET Segmentation/Paging: SEG + PT_Index + OFFSET Multi-level page tables: PD_Index + PT_Index + OFFSET

16KB address space 64-byte pages 4-byte PTE

>2 Levels Problem: page directories may not fit In a page Solution: split page directories into pieces. Use another page dir to refer to the page dir pieces. Virtual address Basic paging: VPN(PT_Index) + OFFSET Segmentation/Paging: SEG + PT_Index + OFFSET Multi-level page tables: PD_Index0 + …1 + PT_Index + OFFSET

How many levels do we need? 30-bit virtual address space, 512-byte pages, 4-byte PTE How large is the virtual addr space, assuming 4-KB pages and 4-byte entries with N levels? (PT can now no longer require more than 1 contiguous page for bookkeeping) (a) N = 1 (b) N = 2 (c) N = 3

What about TLBs? Lookups in multiple levels more expensive. How much does a miss cost? Time/Space tradeoffs

10 else // TLB Miss 11 // first, get page directory entry 12 PDIndex = (VPN & PD_MASK) >> PD_SHIFT 13 PDEAddr = PDBR + (PDIndex* sizeof(PDE)) 14 PDE = AccessMemory(PDEAddr) 15 if (PDE.Valid == False) 16 RaiseException(SEGMENTATION_FAULT) 17 else 18 // PDE is valid: now fetch PTE from page table 19 PTIndex = (VPN & PT_MASK) >> PT_SHIFT 20 PTEAddr = (PDE.PFN << SHIFT) + (PTIndex*sizeof(PTE)) 21 PTE = AccessMemory(PTEAddr) 22 if (PTE.Valid == False) 23 RaiseException(SEGMENTATION_FAULT) 24 else if (CanAccess(PTE.ProtectBits) == False) 25 RaiseException(PROTECTION_FAULT) 26 else 27 TLB_Insert(VPN, PTE.PFN, PTE.ProtectBits) 28 RetryInstruction()

How Many Physical Accesses Assume a 3-level page table Assume 256-byte pages, 16-bit addresses. Assume ASID of current process is 211 0xAA10: movl 0x1111, %edi 0xBB13: addl $0x3, %edi 0x0519: movl %edi, 0xFF10 validVPNPFNASIDProt 10xBB0x91211? 10xFF0x23211? 10x050x91112? 00x050x12211?

Summary Many PT options are possible … Inverted page table Mixed-size page table Time/Space/Complexity tradeoffs

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