One of the most anticipated features of Sun’s ROCK processor was Transactional Memory, which was intended to simplify parallel programming by allowing a group of load and store instructions to execute in an atomic way. Now, some 18 months after Oracle cancelled ROCK, IBM has become the first company to ship a commercial microprocessor using transactional memory.
According an IBM paper published at the recent Hot Chips conference, the BlueGene/Q processor used in the Sequoia supercomputer IBM is building for Lawrence Livermore National Labs will employ the new feature. Sequoia is expected to deliver 20 petaflops when it is complete in 2012.
IBM only implemented transactional memory within the confines of a single chip using a tagging scheme on the chip’s level-two cache memory. The tags are used to detect any load/store conflicts in data to be used in a so-called atomic transaction scheduled by the computer. If no conflicts are found, the job can be processed. If conflicts do appear, the chip asks system software to resolve them.
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