Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 191Lecture 7: Lazy & Eager Transactional Memory• Topics: details of “lazy” TM, scalable lazy TM, implementation details of eager TM2Lazy OverviewTopics:• Commit order• Overheads• Wback, WAR, WAW, RAW• Overflow• Parallel Commit• Hiding Delay• I/O• Deadlock, Livelock, StarvationCPR WCPR WCPR WCPR WMA3“Lazy” Implementation (Partially Based on TCC)• An implementation for a small-scale multiprocessor with a snooping-based protocol• Lazy versioning and lazy conflict detection• Does not allow transactions to commit in parallel4Handling Reads/Writes• When a transaction issues a read, fetch the block in read-only mode (if not already in cache) and set the rd-bit for that cache line• When a transaction issues a write, fetch that block in read-only mode (if not already in cache), set the wr-bit for that cache line and make changes in cache• If a line with wr-bit set is evicted, the transaction must be aborted (or must rely on some software mechanism to handle saving overflowed data) (or must acquire commit permissions)5Commit Process• When a transaction reaches its end, it must now make its writes permanent• A central arbiter is contacted (easy on a bus-based system), the winning transaction holds on to the bus until all written cache line addresses are broadcasted (this is the commit) (need not do a writeback until the line is evicted or written again – must simply invalidate other readers of these lines)• When another transaction (that has not yet begun to commit) sees an invalidation for a line in its rd-set, it realizes its lack of atomicity and aborts (clears its rd- and wr-bits and re-starts)6Miscellaneous Properties• While a transaction is committing, other transactions can continue to issue read requests• Writeback after commit can be deferred until the next write to that block• If we’re tracking info at block granularity, (for various reasons), a conflict between write-sets must force an abort7Summary of Properties• Lazy versioning: changes are made locally – the “master copy” is updated only at the end of the transaction• Lazy conflict detection: we are checking for conflicts only when one of the transactions reaches its end• Aborts are quick (must just clear bits in cache, flush pipeline and reinstate a register checkpoint)• Commit is slow (must check for conflicts, all the coherence operations for writes are deferred until transaction end)• No fear of deadlock/livelock – the first transaction to acquire the bus will commit successfully• Starvation is possible – need additional mechanisms8TCC Features• All transactions all the time (the code only defines transaction boundaries): helps get rid of the baseline coherence protocol• When committing, a transaction must acquire a central token – when I/O, syscall, buffer overflow is encountered, the transaction acquires the token and starts commit• Each cache line maintains a set of “renamed bits” – this indicates the set of words written by this transaction – reading these words is not a violation and the read-bit is not set9TCC Features• Lines evicted from the cache are stored in a write buffer; overflow of write buffer leads to acquiring the commit token• Less tolerant of commit delay, but there is a high degree of “coherence-level parallelism”• To hide the cost of commit delays, it is suggested that a core move on to the next transaction in the meantime – this requires “double buffering” to distinguish between data handled by each transaction• An ordering can be imposed upon transactions – useful for speculative parallelization of a sequential program10Parallel Commits• Writes cannot be rolled back – hence, before allowing two transactions to commit in parallel, we must ensure that they do not conflict with each other• One possible implementation: the central arbiter can collect signatures from each committing transaction (a compressed representation of all touched addresses)• Arbiter does not grant commit permissions if it detects a possible conflict with the rd-wr-sets of transactions that are in the process of committing• The “lazy” design can also work with directory protocols11Scalable Algorithm – Lazy Implementation• Data is distributed across several nodes/directories• Each node has a token• For a transaction to commit, it must first acquire all tokens corresponding to the data in its read and write set – this guarantees that an invalidation will not be received while this transaction commits• After performing the writes, the tokens are released• Tokens must be acquired in numerically ascending order for deadlock avoidance – can also allow older transactions to steal from younger transactions12ExampleP1T1D1: X ZP2T2 YRd XWr XRd YWr ZD2:13“Eager” OverviewTopics:• Logs• Log optimization• Conflict examples• Handling deadlocks• Sticky scenarios• Aborts/commits/parallelismCDirPR WCDirPR WCDirPR WCDirPR WScalable Non-broadcast Interconnect14“Eager” Implementation (Based Primarily on LogTM)• A write is made permanent immediately (we do not wait until the end of the transaction)• Can’t lose the old value (in case this transaction is aborted) – hence, before the write, we copy the old value into a log (the log is some space in virtual memory -- the log itself may be in cache, so not too expensive) This is eager versioning15Versioning• Every overflowed write first requires a read and a write to log the old value – the log is maintained in virtual memory and will likely be found in cache • Aborts are uncommon – typically only when the contention manager kicks in on a potential deadlock; the logs are walked through in reverse order• If a block is already marked as being logged (wr-set), the next write by that transaction can avoid the re-log• Log writes can be placed in a write buffer to reduce contention for L1 cache ports16Conflict Detection and Resolution• Since Transaction-A’s writes are made permanent rightaway, it is possible that another Transaction-B’s rd/wr miss is re-directed to Tr-A• At this point, we detect a conflict (neither transaction has reached its end, hence, eager conflict detection): two transactions handling the same cache line and