Web Caching and Content Distribution: A View From the Interior

Transcription

1 Web achg ad otet Dstrbuto: A Vew From the Iteror Syam Gadde Jeff hase Duke Uversty Mchael AT&T Labs - Research Overvew Aalytcal tools have evolved to predct behavor of large-scale Web caches. Are results from exstg large-scale caches cosstet wth the predctos? ANR What do the models predct for otet Dstrbuto/Delvery Networks (DNs? Goal: aswer these questos by extedg models to predct teror cache behavor. Geeralzed ache/dn (Exteral Vew Geeralzed ache/dn (Iteral Vew Org DNs Web aches {push, request,reply} Request Routg Fucto ƒ ƒ Iteror aches root caches reverse proxes lets {request, reply} boud clet populatos Goals ad Lmtatos Outle Focus o teror cache behavor. Assume leaf caches are ubqutous. Model DNs as teror caches. Focus o ht rato (percetage of accesses absorbed by the cloud. Igore push replcato; at best t merely reduces some lateces by movg data earler. Focus o typcal statc Web objects. Igore streamg meda ad dyamc cotet. Aalytcal model appled to teror odes of cache herarches appled to DNs Implcatos of the model for DNs the presece of ubqutous leaf cachg Match model wth observatos from the ANR cache herarchy ocluso Vew From the Iteror

2 Aalytcal Model [Wolma/Voelker/Levy et. al., SOSP 999] refes [Breslau/ao et. al., 999], ad others Approxmates asymptotc cache behavor assumg Zpf-lke object popularty caches have suffcet capacty Parameters: λ per-clet request rate µ rate of object chage p c percetage of objects that are Zpf parameter (object popularty acheable Ht Rato: the Formula N s the ht rato for objects achevable by populato of sze N wth a uverse of objects. N x µ x + λ N x [Wolma/Voelker/Levy et. al., SOSP 99] Isde the Ht Rato Formula Approxmates a sum over a uverse of objects......of the probablty of access to each object x... tmes the probablty x was accessed sce ts last chage. A Idealzed Herarchy Level (Root N s just a ormalzg costat for the Zpf-lke popularty dstrbuto (a PDF. x µ x + λn λ x /Ω [Breslau/ao 99] 0<< N 2 clets N 2 clets N clets Level 2 Assume the trees are symmetrc to smplfy the math. Igore dvdual caches ad solve for each level. Ht Rato at Iteror Level Root Ht Rato N gves us the ht rato for a complete subtree coverg populato N The ht rato predcted at level or at ay cache level s gve by: hts at level requests to level h Rpc ( N N + r r + h + Predcted ht rato for objects, observed at root of a two-level cache herarchy (.e. where r 2 Rp c : h N N2 r N 2 the hts for N (at level mus the hts captured by level +, over the mss stream from level + Vew From the Iteror 2

3 Geeralzg to DNs Request Routg Fucto ƒ(leaf, object, state ƒ Iteror aches (supply sde N I clets DN Iteror aches DN2 N L clets N L clets N clets (demad sde N L clets Symmetry assumpto: ƒ s stable ad balaced. Iteror aches What happes to N f we partto the object uverse? N I clets N I clets Vew From the Iteror 3

4 Ht rato DN caches Gve the symmetry ad balace assumptos, the ht rato at the teror (DN odes s: DN DN2 NI N I s the covered populato at each DN cache. N L s the populato at each leaf cache. Aalyss Aalyss (cot d We apply the model to ga sght to teror cache behavor wth: varyg leaf cache populatos (N L e.g., bgger leaf caches varyg rato of teror to leaf cache populatos (N I /N L e.g., more specalzed teror caches Zpf parameter chages e.g., more cocetrated popularty Fxed parameters (uless oted otherwse: λ (clet request rate 590 reqs./day µ (rate of object chage oce every 4 days (popular objects, 0.3% oce every 86 days (upopular objects p c (percet of requests 60% (Zpf parameter - object popularty 0.8 Vew From the Iteror 4

5 acheable teror ht rato observed at teror level fxg teror/leaf populato rato Iteror ht rato as percetage of all requests, fxg teror/leaf populato rato ht rato margal ht rato creasg N I ad N L --> creasg N I ad N L --> acheable teror ht rato fxg leaf populato acheable teror ht rato as percetage of all requests fxg leaf populato ht rato margal ht rato creasg bushess --> creasg bushess --> acheable teror ht rato as percetage of all requests varyg Zpf parameter acheable teror ht rato as percetage of all requests varyg Zpf parameter ht rato ht rato N L fxed at 024 clets N I /N L fxed at 64K Vew From the Iteror 5

6 oclusos (I Iteror ht rato captures effectveess of upstream caches at reducg access traffc fltered by leaf/edge caches. Ht ratos grow rapdly wth covered populato. Edge cache populatos (N L are key: s t oe thousad or oe mllo? Wth large N L, teror ratos are deceptve. At N L 0 5, teror ht ratos mght be 90%, but the DN sees less tha 20% of the requests. orrelatg wth ANR Observatos Do the predctos match observatos from exstg large-scale caches? Observatos made from traces provded by ANR (0/2/99. Observed total ht rato at (ufed root s 32% 200 of the 94 leaf caches the trace accout for 95% of requests daly request rate dcates populato s o the order of tes of thousads What s the predcted N? Model vs. Realty ANR roots cooperate; we flter the traces to determe the ufed root ht rato. ANR caches are bouded; traces mply that capacty msses are low at 6GB. Aalyss assumes the populato s balaced across the 200 leaves of cosequece. Aalyss must compesate for objects determed to be u at a leaf. acheable teror ht rato varyg percetage of requests detected as u by leaves ht rato 200+ leaf caches acheable teror ht rato varyg percetage of requests detected as u at request tme oclusos (II ht rato 000 clets per leaf cache ANR root effectveess s aroud 32% today; t s servg ts users well. ANR expermet could valdate the model, but more data from the expermet s eeded. E.g., covered populatos, leaf summares The model suggests that the populato covered by ANR s relatvely small. Wth larger N ad N L, hgher root ht ratos are expected, wth lower margal beeft. Vew From the Iteror 6

7 Modelg DNs If the routg fucto satsfes three propertes: a teror cache sees all requests for each assged object x from a populato of sze N I every teror cache sees a equvalet object popularty dstrbuto (/λ held costat all requests are routed through leaf caches that serve N L clets the teror ht rato s: NI Ht rato wth detected u documets p u s the percetage of u requests detected at request tme (ad ot forwarded to parets: Rpc ( N N + R h + ( pc ( pu r + h r h r 2 HN2 H H N N2 ( p ( p c u ache Herarches As troduced by the Harvest project k levels of demad-sde caches arraged a tree (for ow clets are boud to leaves each ode s mss stream routes to ts paret As exteded by ANR (Squd ANR-operated root caches cooperate by parttog URL space ache Herarches Illustrated Level (Root Level 2 Vew From the Iteror 7