Project: Translation Look-Aside Buffer

1. Project: Translation Look-Aside Buffer ICS 143B L. Bic NOTE: This is an individual project—no groups are allowed

2. Assignment • implement a software emulation of a TLB • generate reference strings • study how TLB reduces number of main memory references

3. Organization of VM and TLB • assume both paging and segmentation (Sect. 8.2.3) • VM address is an integer, divided into s,p,w • TLB:

4. Organization of VM and TLB • implement TLB as an integer array: • TLB.LRU[], TLB.SP[] • frame number f is not needed • implement each s,p as a single integer • operations on TLB: • do for each s,p from a given RS: • search TLB.SP[] sequentially for a match • if not found, replace one entry with the current s,p using LRU information • if found, count as TLB hit • update LRU information

5. Organization of VM and TLB • LRU implementation • LRU field is a counter c • for any given sp: • when sp is found (TLB hit), set corresponding c=0 • increment c of all frames by 1 • when sp is not found (TLB miss), select entry with largest c for replacement

6. Performance evaluations • measure effectiveness of TLB: • disregards page faults (assume VM is resident) • disregard caching • cost w/o TLB: 3 main memory accesses (3m) • cost w/ TLB: • fixed overhead for each VM access, cm • memory accesses: 1m for each TLB hit, 3m for each TLB miss • determine hit ratio h experimentally cost of VM access without TLB cost of VM access with TLB E =

7. Performance evaluations • determine best case and worst case (h=0, h=1, c=0, c=1) • determine best case and worst case for realistic c (e.g., c=0.1) • determine h using simulation for different RS types; compute and plot E 3m hm+(1–h)3m+cm 33–2h+c E = =

8. Performance evaluations • determine h for different RSs and TLB sizes hit_cnt=0; for (i=0; i<length_of_RS; i++) { read next element sp of RS; search for i such that TLB.SP[i]==sp; if (no such i exists) //TLB miss find i such that TLB.LRU[i] is max; TLB.SP[i]=sp; else hit_cnt++; TLB.LRU[i]=0; for (j=0; j<length_of_TLB; j++) if (j != i) TLB.LRU[j]++; } h = hit_cnt/length_of_RS;

9. Performance evaluations • variables to experiment with: • program locality • choose different parameters when generating RS • size n of TLB • minimum: n=0 (no TLB) • maximum: use observed h as a guide; when h approaches 1, increasing n further will not help • fixed cost overhead c of using TLB • determine and plot: • hit ratio h • effectiveness E

10. Generating reference strings (analogous to page replacement project) • no locality: pick random number in [0..P-1] • typical behavior: • periods of stable WS • punctuated by transitions: WS grows rapidly, then settles into new stable size

11. Generating reference strings • model typical behavior: • locus of reference  |---------|-------|------------------------------------| 0 sp sp+e P-1 • stable period: • assume constant rate in one direction (1 step every m references) • transition: • generate new locus with probability t • locus defined by sp, e, m, t

12. Generating reference strings • Note: algorithm for generating RS is on page 498/499 (page replacement project) • for TLB project, pages are grouped into segments but we do not need to choose s and p separately; an integer sp in the range 0:P-1 represents a page

13. Generating reference strings • algorithm to generate RS: • select P, sp, e, m, t • repeat until RS generated: • pick m random numbers in [sp..sp+e]; include in RS • generate random number 0 <= r <= 1; • if (r < t) generate new sp else increment sp (mod P)

14. Choosing simulation constants • P: size of VM (# pages) • e: size of working set (# pages) • P and e need to be chosen together: e<P, but: • if e is too close to P, working sets overlap after transitions • if e is too small, the program will rarely revisit previous locations (important for LRU to benefit) • suggestion: e varies from 2 to P/10, P=500 or 1000 • sp random number within [0..P-1]

15. Choosing simulation constants • m: # of times a page is referenced • typically, m100 (see page 267, fig 8-13b) • t: length of stable period • random number in [0..1]; typically, t<0.1 (i.e., thousands of instructions are executed between transitions) • length of RS: • must be large to make behavior statistically significant • suggestion: 100,000

16. Summary of tasks • develop simulation program • generate representative reference strings • run simulation program: • for each type of RS, • vary TLB size n and overhead value c • record hit ratio h • compute corresponding effectiveness E • plot h and E over TLB size for different c values