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15-213 Recitation 7 Greg Reshko

15-213 Recitation 7 Greg Reshko. Office Hours: Wed 2:00-3:00PM March 31 st , 2003. Outline. Virtual Memory Paging Page faults TLB Address translation Malloc Lab Lots of hints and ideas. Virtual Memory. Reasons Use RAM as a cache for disk Easier memory management Protection

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15-213 Recitation 7 Greg Reshko

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  1. 15-213 Recitation 7Greg Reshko Office Hours: Wed 2:00-3:00PM March 31st, 2003

  2. Outline • Virtual Memory • Paging • Page faults • TLB • Address translation • Malloc Lab • Lots of hints and ideas

  3. Virtual Memory • Reasons • Use RAM as a cache for disk • Easier memory management • Protection • Enable ‘partial swapping’ • Share memory efficiently

  4. CPU Physical memory Memory 0: Physical Addresses 1: N-1:

  5. 0: 1: CPU N-1: Virtual Memory Memory Page Table Virtual Addresses Physical Addresses 0: 1: P-1: Disk

  6. Paging: Purpose • Solves two problems • External memory fragmentation • Long delay to swap a whole process • Divide memory more finely • Page – small logical memory region • Frame – small physical memory region • Any page can map to any frame

  7. Paging: Address Mapping Logical Address Page Offset Frame Offset .... f29 f34 .... Physical Address Page table

  8. Paging: Multi-Level .... f99 f87 .... P1 P2 Offset Frame Offset .... f07 f08 .... .... f29 Page Directory f34 f25 Page Tables

  9. Page Faults • Virtual address not in memory • This means it is on a disk • Go to disk, fetch the page, load it into memory, get back to the process Memory Memory Page Table Page Table Virtual Addresses Physical Addresses Virtual Addresses Physical Addresses CPU CPU Disk Disk

  10. Copy-on-Write • “Simulated” Copy • Copy page table entries to new process • Mark PTEs read-only in old and new • What really happens • Process writes to page • Page fault handler is called • Copy page into empty frame • Mark read-write in both PTEs • Result • Faster and less work

  11. Relevance to Fork • Why is paging good for fork and exec? • Fork produces two very similar processes • Same code, data, and stack • Copying all pages is expensive • Many will never be modified (especially in exec) • Share pages instead • i.e. just mark them as read only and duplicate when necessary

  12. Address Translation:General Idea • Mapping between virtual and physical addresses page fault fault handler Processor  Hardware Addr Trans Mechanism Secondary memory Main Memory V P OS performs this transfer (only if miss) virtual address part of the on-chip memory mgmt unit (MMU) physical address

  13. Address Translation: In terms of address itself • Higher bits of the address get mapped from virtual address to physical. • Lower bits (page offset) stays the same. p p–1 0 n–1 virtual address virtual page number page offset address translation m–1 p p–1 0 physical page number page offset physical address

  14. hit miss VA PA TLB Lookup Cache Main Memory CPU miss hit Trans- lation data TLB • Translation Lookaside Buffer • Small hardware cache in MMU • Maps virtual page numbers to physical page numbers

  15. Address Translation with TLB n–1 p p–1 0 virtual address virtual page number page offset valid tag physical page number TLB . . . = TLB hit physical address tag byte offset index valid tag data Cache = data cache hit

  16. Example • Motivation: • A detailed example of end-to-end address translation • Same as in the book and lecture • I just want to make sure it makes perfect sense • Do practice problems at home • Ask questions if anything is unclear

  17. Example: Description • Memory is byte addressable • Accesses are to 1-byte words • Virtual addresses are 14 bits • Physical addresses are 12 bits • Page size is 64 bytes • TLB is 4-way set associative with 16 total entries • L1 d-cache is physically addressed and direct mapped, with 4-byte line size and 16 total sets

  18. 9 2 13 12 11 10 9 8 7 5 4 3 0 6 11 0 1 2 3 4 1 6 7 8 10 5 PPO VPN VPO PPN Example: Addresses • 14-bit virtual addresses • 12-bit physical address • Page size = 64 bits (Virtual Page Offset) (Virtual Page Number) (Physical Page Number) (Physical Page Offset)

  19. Example: Page Table …

  20. TLBI TLBT 7 13 12 11 10 9 8 5 4 3 2 1 0 6 VPO VPN Example: TLB • 16 entries • 4-way associative

  21. CO CI CT 6 11 10 9 8 7 5 3 2 1 0 4 PPO PPN Example: Cache • 16 lines • 4-byte line size • Direct mapped

  22. Example: Address Translation • Virtual Address 0x03D4 • Split into offset and page number • 0x03D4 = 00001111010100 • VPO = 010100 = 0x14 • VPN = 00001111 = 0x0F • Lets see if this is in TLB • 0x03D4 = 00001111010100 • TLBI = 11 = 0x03 • TLBT = 000011 = 0x03

  23. TLBI TLBT 7 13 12 11 10 9 8 5 4 3 2 1 0 6 VPO VPN Example: TLB • 16 entries • 4-way associative

  24. Example: Address Translation • Virtual Address 0x03D4 • TLB lookup • This address is in TLB (second entry, set 0x3) • PPN = 0x0D = 001101 • PPO = VPO = 0x14 = 010100 • PA = PPN + PPO = 001101010100 • Cache • PA = 0x354 = 0x001101010100 • CT = 001101 = 0x0D • CI = 0101 = 0x05 • CO = 00 = 0x0

  25. CO CI CT 6 11 10 9 8 7 5 3 2 1 0 4 PPO PPN Example: Cache • 16 lines • 4-byte line size • Direct mapped

  26. Example: Address Translation • Virtual Address 0x03D4 • Cache Hit • Tag in set 0x5 matches CT • Data at offset CO is 0x36 • Data returned to MMU • Data returned to CPU

  27. Lab 6 Hints and Ideas • Due April 16 • 40 points for performance • 20 points for correctness • 5 points for style • Get the correctness points this week • Get a feel for how hard the lab is • You'll probably need the time • Starting a couple days before is a BAD idea!

  28. How to get the correctness points • We provide mm-helper.c which contains the code from the book • malloc works • free works (with coalescing) • Heap checking doesn't work • realloc doesn't work • Implement a dumb version of realloc • malloc new block, memcpy, free old block, return new block

  29. How to get the correctness points • Implement heap checking • Have to add a request id field to each allocated block (tricky) • Hint: need padding to maintain 8 byte alignment of user pointer • In the book's code bp always the same as the user pointer • The 4 bytes immediately before bp contain size of payload • 3 lsb of size unused (because of alignment) • first bit indicates of the block is alloced or not Size+a Payload… Footer bp

  30. How to get the correctness points • Need to change block layout to look like this: • This changes how the implicit list has to be traversed • But size is at same place relative to bp ID Size+a Payload… Footer bp

  31. How to get the correctness points • Or change block layout to look like this: • All accesses to what was size now access id but can be clever and make size 4 bytes larger • Could even make bp point to id.. • Most code would just work Size+a ID Payload… Footer bp

  32. How to get the correctness points • Once malloc, free, and realloc work with the id field, write heapcheck • Iterate over the whole heap and print out allocated blocks • Need to read the id field… • That's it for correctness

  33. Hints • Remember that pointer arithematic behaves differently depending on type of pointer • Consider using structs/unions to eliminate some messy pointer code • Get things working with the short trace file first:./mdriver -f short1-bal.rep • To get the best performance • Red-Black trees • Ternary trees • Other interesting data structures

  34. That’s it for hints… Good Luck!

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