SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION MARC S. ORR †§, SHUAI CHE §, AYSE YILMAZER §, BRADFORD M. BECKMANN §, MARK D. HILL †§, DAVID A. WOOD †§ † UW-MADISON, § AMD RESEARCH ASPLOS, MARCH 16, 2015

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16, EXECUTIVE SUMMARY All Global Synchronization Scoped Synchronization Work Stealing Best of Both? NEW: Remote-Scope Promotion (7% Speedup)(18% Speedup) (25% Speedup) Heterogeneous chips, like GPUs, have hierarchical memories

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16, OUTLINE  Background: Synchronization + Scopes  Synchronization using Remote-Scope Promotion  Results/Conclusion

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16, BACKGROUND: SYNCHRONIZATION + SCOPES  Parallel Synchronization semantics ‒acquire: pull latest data (to me) ‒release: push latest data (to others)  Scopes bound synchronization: ‒Smaller scope  less synchronization overhead

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16, wg scope0wg scope1 ACQUIRE/RELEASE ANIMATION void incX_ workgroup () { } while (!CAS_ acq_wg (&L, 0, 1)); X = X + 1; st_ rel_wg (&L, 0); void incX_ component () { } while (!CAS_ acq_cmp (&L, 0, 1)); X = X + 1; st_ rel_cmp (&L, 0); component scope L1 Cache L2 X = 2 L = 0 CU0CU1 X = 1 L = 1 X = 3 L = 0 L = 1 X = 4 L = 0

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16, SCOPED SYNCHRONIZATION’S STRENGTHS Static local sharingDynamic global sharing component scope wg_scope0 data 0 wg_scope1 data 1 wg scope0 global data store wg scope1 On current hardware, wg scope can yield >20% speedup over cmp scope

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16, SCOPED SYNCHRONIZATION’S LIMITATIONS  Dynamic local sharing: some threads access shared data less frequently than others in an ad-hoc manner  Example: work stealing component scope queue 0 stale wg scope0 wg scope1 queue 1 enq deq queue 0

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16, OUTLINE  Background: Synchronization + Scopes  Synchronization using Remote-Scope Promotion  Results/Conclusion

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16, REMOTE-SCOPE PROMOTION  Insight: wg1 needs to trigger the promotion of scope 0  Contribution: hardware support for scope promotion & ISA instructions that utilize it component scope queue 0 stale wg_scope0 wg_scope 1 queue 1 queue 0 promote flush deq queue 0

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16,  Prior memory models: HRF-direct, HRF-indirect ‒Invariant: acquire/release pair must occur at the same scope  Three new memory orders: st_rel_cmp(L, 0) PROMOTION SEMANTIC st(V,2) st_rel_wg(L, 0) cas_acq_wg(&L, 0, 1) ld(R1, V) work-item 0 (in wg 0)work-item 1 (in wg 1) OK cas_acq_cmp(&L, 0, 1) RACE! cas_rm_acq_cmp(&L, 0, 1) OK synchronizes-with relationship promotion remoteAcquirePromote the scope of last release to the scope of this acquire, then perform acquire remoteReleasePromote the scope of next acquire to the scope of this release, then perform release remoteAcquire+Releasecombine remote acquire & remote release st_rel_wg(L, 0)

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16, IMPLEMENTATION  remote_acq_cmp(L)  remote_rel_cmp(L) CU0 L1 Cache CU1 L1 Cache L2 L = 1 V = 3 V = 2 CU2 L1 Cache FLUSH V = 3 FLUSH L = 0 L = 1 promote 1.Promote the scope of the last release on L 2.Perform an acquire operation on L 1.Perform a release operation on L 2.Promote the scope of the next acquire on L

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16, IMPLEMENTATION DETAILS  Hardware Support ‒Sending/receiving sub-operations between CUs ‒Cache line locking to resolve races  Guarantee “coherence order” for read-modify-writes ‒Hardware support to stall new synchronization operations at target scope  Paper formalizes scope promotion ‒Shows that scope promotion is compatible with coherence order

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16, OUTLINE  Background: Synchronization + Scopes  Synchronization using Remote-Scope Promotion  Results/Conclusion

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16, METHODOLOGY  Prototyped remote scoped synchronization in gem5 ‒Extended with internal GPU model  Refactored 3 Pannotia workloads to retrieve graph nodes from task queues ‒SSSP, Color, PageRank (each run with 3-4 inputs)

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16, RESULTS scenarioScope of sync.?Work stealing? 1.07x1.18x1.25x baselineglobalno scope-onlylocalno steal-onlyglobalYes rem-synclocalYes

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16, CONCLUSION All Global Synchronization Scoped Synchronization Work Stealing NEW: Remote-Scope Promotion (7% Speedup)(18% Speedup) (25% Speedup) Best of Both!

Questions?

Backup

| SYNCHRONIZATION USING REMOTE-SCOPE PROMOTION | MARCH 16, µ BENCHMARK RESULTS  Scopes matter! Small tasks benefit from scopes

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