1. CS703 - Advanced Operating Systems
By Mr. Farhan Zaidi

2. Lecture No. 28

3. Page Fault Frequency (PFF)
PFF is a variable-space algorithm that uses a more ad hoc approach
Attempt to equalize the fault rate among all processes, and to have a “tolerable” system-wide fault rate
monitor the fault rate for each process
if fault rate is above a given threshold, give it more memory
so that it faults less
if the fault rate is below threshold, take away memory
should fault more, allowing someone else to fault less

4. Fault resumption. lets us lie about many things
Emulate reference bits:
Set page permissions to “invalid”.
On any access will get a fault: Mark as referenced
Emulate non-existent instructions:
Give inst an illegal opcode. When executed will cause “illegal instruction” fault. Handler checks opcode: if for fake inst, do, otherwise kill.
Run OS on top of another OS!
Make OS into normal process
When it does something “privileged” the real OS will get woken up with a fault.
linux
linux
win98
linux
privileged

8. external system bus (e.g. PCI)
32 bit address space
4 KB page size
L1, L2, and TLBs
4-way set associative
inst TLB
32 entries
8 sets
data TLB
64 entries
16 sets
L1 i-cache and d-cache
16 KB
32 B line size
128 sets
L2 cache
unified
128 KB -- 2 MB
P6 Memory System
DRAM
external system bus (e.g. PCI) L2
cache
cache bus
bus interface unit
inst
TLB
data
TLB
instruction
fetch unit L1
i-cache L1
d-cache
processor package

9. P6 2-level Page Table Structure
Page directory
byte page directory entries (PDEs) that point to page tables
Page tables:
byte page table entries (PTEs) that point to pages.
Up to 1024 page tables
1024
PTEs
page directory
...
1024
PDEs
1024
PTEs
...
1024
PTEs

10. Translating with the P6 TLB
1. Partition VPN into TLBT and TLBI.
2. Is the PTE for VPN cached in set TLBI?
3. Yes: then build physical address.
4. No: then read PTE (and PDE if not cached) from memory and build physical address.
CPU
virtual address 20 12
VPN
VPO 16 4
TLBT
TLBI 1 2
TLB
hit
TLB
miss
PDE
PTE 3
... 20 12
PPN
PPO
physical address
page table translation 4