1、MPICH-V: Fault Tolerant MPI,Rachit Chawla,Outline,Introduction Objectives Architecture Performance Conclusion,Fault Tolerant Techniques,Transparency User Level Re-launch application from previous coherent snapshot API Level Error codes returned to be handled by programmer Communication Library Level
2、 Transparent Fault Tolerant communication layer Checkpoint Co-ordination Coordinated Uncoordinated Message Logging Optimistic - All events are logged in the volatile memory Pessimistic - All events are logged on a stable storage,Fault Tolerant Techniques,Transparency User Level Re-launch application
3、 from previous coherent snapshot API Level Error codes returned to be handled by programmer Communication Library Level Transparent Fault Tolerant communication layer Checkpoint Co-ordination Coordinated Uncoordinated Message Logging Optimistic - All events are logged in the volatile memory Pessimis
4、tic - All events are logged on a stable storage,Checkpointing,Coordinated Coordinator initiates a checkpoint No Domino Effect Simplified Rollback Recovery Uncoordinated Independent checkpoints Possibility of Domino Effect Rollback Recovery Complex,Logging,Piece-Wise Deterministic (PWD) For all Non-D
5、eterministic events, store information in determinant replay Non-Deterministic Events Send/Receive message Software Interrupt System Calls Replay Execution Events after last checkpoint,Objectives,Automatic Fault Tolerance Transparency (for programmer/user) Tolerate n faults (n, # of MPI Processes) S
6、calable Infrastructure/Protocol No Global Synchronization,MPICH-V,Extension of MPICH Comm Lib level Implements all comm subroutines in MPICH Tolerant to volatility of nodes Node Failure Network Failure Uncoordinated Checkpointing Checkpointing Servers Distributed Pessimistic Message Logging Channel
7、Memories,Architecture,Communication Library Relink the application with “libmpichv” Run-Time Environment Dispatcher Channel Memories - CM CheckPointing Servers - CS Computing/Communicating Nodes,Node,Network,Node,Dispatcher,Node,Checkpoint server,Firewall,Firewall,1,2,3,4,Big Picture,Channel Memory,
8、Overview,Channel Memory Dedicated Nodes Message tunneling Message Repository Node Home CM Send a message send to receivers home CM Distributed Checkpointing/Logging Execution Context - CS Communication context - CM,Dispatcher (Stable),Initializes the execution A Stable Service Registry (centralized)
9、 started Providing services - CM, CS to nodes CM, CS assigned in a round-robin fashion Launches the instances of MPI processes on Nodes Monitors the Node state alive signal, or time-out Reschedules tasks on available nodes for dead MPI process instances,Steps,When a node executes Contacts Stable Ser
10、vice Registry Gets assigned CM, CS based on rank Sends “alive” signal periodically to dispatcher contains rank On a failure Restart its execution Other processes unaware about failure CM allows single connection per rank If faulty process reconnects, error code returned, it exits,Channel Memory (Sta
11、ble),Logs every message Send/Receive Messages GET & PUT GET & PUT are transactions FIFO order maintained each receiver On a restart, replays communications using CMs,node,Get,Network,Put,Get,node,Channel Memory,node,Checkpoint Server (Stable),Checkpoint stored on stable storage Execution node perfor
12、ms a checkpoint, send image to CS On a Restart Dispatcher informs about task CS to contact to get last task chkptNode Contacts CS with its rank Gets last chpkt image back from CS,Putting it all together,0,1,2,2,1,CM,CM,CS,1,2,2,Worst condition: in-transit message + checkpoint,Pseudo time scale,Proce
13、sses,Ckpt image,Ckpt image,Ckpt images,Performance Evaluation,xTremeWeb P2P Platform Dispatcher Client excute parrallel application Workers MPICH-V nodes, CMs & CSs 216 PIII 733 Pcs Connected by Ethernet Simulate Node volatility enforce process crashes NAS BT benchmark simulated Computational Fluid
14、Dynamics Application Parallel Benchmark Significant Communication + Computation,Effects of CM on RTT,Time, sec,Mean over 100 measurements,0,0.05,0.1,0.15,0.2,P4,ch_cm 1 CM out-of-core,ch_cm 1 CM in-core,ch_cm 1 CM out-of-core best,X 2,10.5 MB/s,5.6 MB/s,Message size,0,64kB,128kB,192kB,256kB,320kB,38
15、4kB,Impact of Remote Checkpointing,+25%,+2%,+14%,+28%,50,44,78,62,214,208,1.8,1.4,0,50,100,150,200,250,bt.W.4 (2MB),bt.A.4 (43MB),bt.B.4 (21MB),Dist. Ethernet 100BaseT,Local (disc),Cost of remote checkpoint is close to the one of local checkpoint (can be as low as 2%) because compression and transfe
16、r are overlapped,Time between reception of a checkpoint signal and actual restart: fork, ckpt, compress, transfer to CS, way back, decompress, restart,RTT Time, sec,bt.A.1 (201MB),Performance of Re-Execution,The system can survive the crash of all MPI Processes,Execution Time Vs Faults,Number of fau
17、lts,Base exec. without ckpt. and fault,0,1,2,3,4,5,6,7,8,9,10,610,650,700,750,800,850,900,950,1000,1050,1100,Total execution time (sec.),Overhead of chkpt is about 23% For 10 faults performance is 68% of the one without fault,1 fault/110 sec.,Conclusion,MPICH-V full fledge fault tolerant MPI environment (lib + runtime). uncoordinated checkpointing + distributed pessimistic message logging. Channel Memories, Checkpoint Servers, Dispatcher and nodes.,
