Computer Architecture Seminar
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The document discusses eliminating redundant computation through data-triggered threads (DTT). DTT proposes spawning a separate thread to handle redundant loads caused by silent stores, which are stores that do not change memory contents. This avoids recomputing values for redundant loads. The programming model places redundant code in a separate thread triggered by a store. The architecture adds hardware tables to manage thread status and queues. The ISA is modified with new instructions like tstore and tspawn to generate and spawn threads.
![Eliminating Redundant Computation Silent Store: Amemory store operation that does not change the contents at that location 20-68% of all stores are silent [Lepakand Lipasti] How about eliminating the entire stream of computation surrounding a silent store!](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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Computer Architecture Seminar
- 1. Data-Triggered ThreadsEliminating Redundant Computation (HPCA 2011) Hung-Wei Tseng and Dean M. Tullsen Department of Computer Science and Engineering University of California, San Diego Seminar by: Naman Kumar for http://carg.uwaterloo.ca
- 2. Eliminating Redundant Computation Silent Store: Amemory store operation that does not change the contents at that location 20-68% of all stores are silent [Lepakand Lipasti] How about eliminating the entire stream of computation surrounding a silent store!
- 3. Eliminating Redundant Computation Redundant loads: silent stores result in redundant loads (last time this load loaded this address, it fetched the same value) SPEC2000 C: 78% of all loads are redundant 50% of all instructions depend on redundant loads
- 5. DTT: Implementation The Programming Model Place redundant computation in a separate thread: Thread is restartable Thread may be aborted/restarted multiple times Thread management is through architectural changes. Easy to verify data races as thread life is between time between triggering store and main thread join point.
- 6. DTT: Implementation The Programming Model Trigger is placed in data section, not code section
- 7. DTT: Implementation The Programming Model Main Thread
- 8. DTT: Implementation The Programming Model Support thread
- 9. DTT: Implementation Architectural Support Following tables are all implemented in hardware Thread registry (table) Thread Queue (table) Thread Status Table (table) PC
- 10. DTT: Implementation Architectural Support ISA modifications tstore – generate thread when mem modified is not silent tspawn – spawn the thread using thread registry treturn– finish execution of the current thread tcancel – terminate a running thread
Editor's Notes
- Memoization and other techniques save on memory access. This technique proposes a solution to save on accesses and the computation involving the data from these access.
- eg: sum of all nodes in a 100 node linked-list. Each node has to be accessed when say, only 2 have changed. That’s 98 redundant loads.
- If value of SP calculated is diff from what is in memory, then a support thread (S) will be spawned to calculate and B. Main thread will skip code section B since data has already been calculated. Instructions for B will be left as is because support thread may have failed to spawn; skipping the thread, code will be executed by the main thread.
- Programmers implement this with C pragma constructs
- Every time the variable is WRITTEN to, the associated DTThread is executed
- If programmer has a reason to suspect that the thread may crash/be aborted, he can place the #cancel pragma. This will ensure that only the main thread executes this block. Support thread will not be registered.
- This function is triggered in a new thread (support thread) when control reaches “#block xxx”
- Start PC: PC of the skippable code in the main thread.Destination PC denotes the end of the skippable region.Post skip PC is address after the region is skipped.
- CMP only