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There are tradeoffs between software TM and HTM. Software TM is more flexible than HTM and it doesnt require specific hardware support (other than a few atomic operations). However, it has a bigger performance overhead than HTM (cache operations are fast). On the other hand, HTM requires specific design to implement it in a system, and is contrained physically, for example by the size and performance of the cache. It therefore makes sense that a lot of systems use a hybrid model.


Also, doing logs within the cache can be bad for performance of the rest of the system. If your transactions are large enough to fill up cache lines, you may need to dump your cache to disk in order to both preserve the log and allow other threads to continue their work.