Overview. Making the Fast Case Common and the Uncommon Case Simple in Unbounded Transactional Memory. Running Example. Background

Transcription

1 Overview king the Fst Cse Common n the Uncommon Cse imple in Unoune Trnsctionl Colin Blunell (University of Pennsylvni) Joe Devietti (University of Pennsylvni) E Christopher Lewis (Vwre, Inc.) ilo. K. rtin (University of Pennsylvni) mll trnsctions: no prolem Implement using locl structures of oune size imple/highly-concurrent/low-overhe Overflowe trnsctions: prolem Difficult to preserve ll nice properties of oune T ny ppers in lst severl yers Previous pproches: focus on concurrency + ustin performnce s overflows increse Involve complex resource mnipultion Our pproch: ecouple into two prolems imple overflow hnling: OneT king overflows rre: Permissions-only cche [ 2 ] Bckgroun unning Exmple Trnsctionl memory: the new hot thing Interfce: seriliztion Implementtion: optimistic prllelism Tsks of every T Conflict etection: ws serilizility violte? Version mngement: how o we recover serilizility? Boune hrwre T implementtion: Conflict etection: exten cche coherence Version mngement: mny schemes tte Dt 56 L1 irect-mppe No L2 Invlition-se system & mp to sme L1 entry [ 3 ] [ 4 ] 1

2 Trnsctionl Execution Conflict Detection lo re tte Dt tte Dt Conflict etection is locl [ 5 ] [ 6 ] Committing Trnsction Version ngement commit store, 42 tte Dt tte Dt Commits re locl 56 + Commits o not chnge + is not oune [ 7 ] [ 8 ] 2

3 Aorting Trnsction The Ctch: Overflows tte Dt re lo tte Dt Nee nother mechnism for conflict etection [ 9 ] [ 10 ] Hnling Overflows: trwmn Hnling Overflows: trwmn lo tte Dt lo tte Dt re re re Preserve sfety [ 11 ] [ 12 ] 3

4 The Ctch to Hnling Overflows The Ctch to Hnling Overflows tte Dt n sets tte Dt unoune Nee mett for ll n processors 56 Nee mett for ll n processors ech thre [ 13 ] [ 14 ] The Ctch to Hnling Overflows est of my tlk: ifferent pproch tte UT, VT, PT, Bulk, T(-E), Dt 56 unoune How to etect conflicts efficiently? How to commit efficiently? How to (e)llocte mett? Clim 1: ouning concurrency of overflows simplifies implementtion Eses the prolem of conflict etection emoves the prolem of ynmic mett lloction Is unoune concurrency necessry? Depens on the frequency of overflows Clim 2: e cn mke overflows rre Tke ech clim in orer Clim 1: OneT Clim 2: Permissions-only cche [ 15 ] [ 16 ] 4

5 OneT Key ie one overflowe trnsction t time On per-ppliction sis Better nme: HighlnerT? Two implementtions OneT-erilize ll thres stll for overflow OneT-Concurrent: serilize only overflows Key mechnism: per-ppliction overflow it Processors check to etermine when to stll Coherently cche in specil register Fully Concurrent OneT-erilize OneT-erilize P2 P3 P2 P3 No chnges to oune T imilr to originl TCC, ut: intin orts tnr CC protocol Non-trns Boune Overflowe tlle Time 4-processor execution No conflicts [ ] [ 18 ] OneT-erilize Evlution OneT-erilize Evlution 8 processors imics + GE Compre to T tht Tkewy ielizes #1: overflow hnling First If overflows worklo re PLAH2 rre, seriliztion is sufficient tree-<n>: mix of uptes & re scns (n% re scns) Performnce worse s numer of overflows increses [ 19 ] [ 20 ] 5

6 OneT-Concurrent OneT-Concurrent Conflict Detection Fully Concurrent OneT-erilize OneT-Concurrent P2 P3 P2 P3 P2 P3 Time lo tte Dt re Non-trns Boune Overflowe tlle 4-processor execution No conflicts 56 [ 21 ] [ 22 ] OneT-Concurrent Conflict Detection OneT-Concurrent Commits lo tte Dt 56 re re [ 23 ] + Preserve sfety Ae mett oune Prolem: ctively clering mett is nsty Commit is now high-overhe opertion olution: lzy clering of mett echnism: overflowe trnsction ID s Block mett extene to inclue ID s Current ID store with overflow it Key: only one ctive ID (so, notion of current ID ) Chnges + Commit now chep iens tpth Amits flse conflicts (since ID s re finite-length) [ 24 ] 6

7 OneT-Concurrent: Evlution The Permissions-Only Cche lo tte Dt re + Performnce etter thn OneT-erilize till flls off iel s overflows increse PO Cche tte E 56 Bck to cche eviction Gol: voi overflow ol n: permissions-only cche [ ] [ 26 ] The Permissions-Only Cche The Permissions-Only Cche lo re tte Dt tte Dt re PO Cche tte E 56 PO Cche tte E 56 Bsiclly unchnge + Conflict etection + Version mngement + Commits & orts [ 27 ] [ 28 ] 7

8 The Permissions-Only Cche The Permissions-Only Cche: Evlution Two key fetures 1. Accesse only on snoops n evictions 2. Efficient encoing (sector cche) Impct: Extens overflow threshol 4 KB PO cche: ~1 B t 64 KB PO cche: ~16 B t tore mett in 4 B L2 t lines: up to 1 GB t Tkewy #2: e cn engineer systems for rre overflows A 4 KB permissions-only cche to OneT [ 29 ] [ 30 ] The Permissions-Only Cche: Evlution Overflows reuce to virtully nil OneT-erilize + PO cche: sweet spot? elte ork Lots! Proposls with low-overhe overflow hnling mechnisms UT/LT, VT, PT, T, Our scheme: PO cche reuces overflow, OneT hnles it simply ny proposls enhnce y permissions-only cche Boune HT s cke y softwre (HyT, T, ) imilr philosophy to ours (uncommon cse simple) Their schemes mintin concurrency ut introuce overhes OneT-Concurrent scrifices concurrency ut hs low overhes Agin, enhnce y permissions-only cche ignture-se Ts: conflict etection through finite-size signtures (Bulk, T-E, ) + igntures cn e sve rchitecturlly + erilize grully rther thn ruptly till n unoune numer of signtures [ ] [ 32 ] 8

9 Conclusions OneT: mke overflow hnling simple OneT-erilize entry-point unoune T OneT-Concurrent: more roust to overflows Permissions-only cche: mke overflows rre + Cn engineer to keep overflow rte low for your worklo + Enhnces mny prior unoune T proposls Comintion: T tht s oth fst n simple to implement [ 33 ] T-E +Very net! Pging more complex thn in OneT Commit of trnsction tht hs migrte processors must trp to O Our hope for PO cche: overflow only on context switch An there T-E loses irectory filter ticky stte + OneT-erilize? Hyri Trnsctionl emories imilr philosophy to OneT Our gol: mke overflows so rre tht it oesn t relly mtter wht you use for them An then OneT-erilize is pretty simple If overflows re frequent, nee to hnle them with high performnce Permissions-only cche + UT/VT/PT? pot in the mile for hyri T s/onet- Concurrent Occsionl overflow: OneT-Concurrent ppeling Tipping point where concurrency mtters more thn overhes I on t know where it is (nee worklos) [ 35 ] [ 36 ] 9

10 Context witching & Pging Context switching just works OneT-erilize overflowe it persists OneT-Concurrent: mett persists s well Pging uring n overflowe trnsction: OneT-erilize no prolem OneT-Concurrent: pge mett (O help) Pging uring oune trnsction: Aort n trnsition to overflowe moe Trnsitioning to Overflowe oe OneT-erilize just set the it ynchronize ccess OneT-Concurrent: hve to set mett imple: ort n restrt (wht we simulte) Higher-performnce schemes re possile lk the cche Overflow grully [ 37 ] [ 38 ] ummry ummry overflow it overflow it tte Dt tte Dt PO Cche tte 56 ett (for OneT-Concurrent only) [ 39 ] [ 40 ] 10

11 The Permissions-only Cche: Efficient torge ector cche to reuce tg overhe Now: (close to) 2 its per t lock 64-yte locks: 6 to 1 compression rtio 4 KB mett 1 B trnsctionl t Even lrger: mett in L2 t lines it to istinguish t/mett 4 B L2: 1 GB trnsctionl t [ 41 ] 11