Lecture 015

Address Space (2)

Translation Lookaside Buffer (TLB)

Problem: multi-layer page table require multiple memory access

Observation: memory access are local

Solution: cache virtual to physical address translation (not data)

One instruction: can have multiple memory access

• Look up next instruction

• Stack pointer access and changing

• Data fetch

• Data store

Typical TLB size: 16~64 entries (bigger size = less efficient), and 2TLB (one for instruction, one for data) is common.

CPU Context Switch:

• restore general purpose registers

• restore page table base register (+length register)

  • when you put new thing to page table base register, x86 will flush (local) TLB for you
  • global pages are not flushed (kernel, shared library, context switch code)

• flush TLB:

TLB Flush: when

• declear new page directory (set %cr3 in x86) flush local TLB

• INVLPG invalidate TLB for specific page

Software-loaded TLB: (more flexible design)

• since OS knows all mapping and permission

• we can let TLB miss generate exception, and let OS handle TLB refill (instead of hardware)

• we can "not lose" certain TLB entires

• we can make TLB faster if impose some constraints on program memory (e.g. same size, then we just do math)

Partial Memory Residence

Benifits:

• error handling code can be loaded only when needed

• tables can ba allocated larger than used

• computer can run program larger than physical memory (as long as you don't need them at once), swapping can't do this

• programs can launch faster

Page Fault Handler

Sometimes, hardware exception can be fixed, othertimes it becomes software exception (illegal memory access)

Software handler: autostack

Page Fault: TLB miss and Page Table not present

Page Fault Handler:

• refill page or kill process (if illegal access)

• make "list of regions" happy by finding "list of pages"

Page Table might not be good data structure for page fault handler:

• per-page nature is repetitive

• disk sector address is 48 bits (> 32 bits)

• per region methods: fault(), evict(), unmap()