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- [/
- Copyright Oliver Kowalke 2017.
- Distributed under the Boost Software License, Version 1.0.
- (See accompanying file LICENSE_1_0.txt or copy at
- http://www.boost.org/LICENSE_1_0.txt
- ]
- [#speculation]
- [section:speculation Specualtive execution]
- [heading Hardware transactional memory]
- With help of hardware transactional memory multiple logical processors
- execute a critical region speculatively, e.g. without explicit
- synchronization.[br]
- If the transactional execution completes successfully, then all memory
- operations performed within the transactional region are commited without any
- inter-thread serialization.[br]
- When the optimistic execution fails, the processor aborts the transaction and
- discards all performed modifications.[br]
- In non-transactional code a single lock serializes the access to a critical
- region. With a transactional memory, multiple logical processor start a
- transaction and update the memory (the data) inside the ciritical region.
- Unless some logical processors try to update the same data, the transactions
- would always succeed.
- [heading Intel Transactional Synchronisation Extensions (TSX)]
- TSX is Intel's implementation of hardware transactional memory in modern Intel
- processors[footnote intel.com: [@https://software.intel.com/en-us/node/695149
- Intel Transactional Synchronization Extensions]].[br]
- In TSX the hardware keeps track of which cachelines have been read from and
- which have been written to in a transaction. The cache-line size (64-byte) and
- the n-way set associative cache determine the maximum size of memory in a
- transaction. For instance if a transaction modifies 9 cache-lines at a
- processor with a 8-way set associative cache, the transaction will always be
- aborted.
- [note TXS is enabled if property `htm=tsx` is specified at b2 command-line and
- `BOOST_USE_TSX` is applied to the compiler.]
- [note A TSX-transaction will be aborted if the floating point state is modified
- inside a critical region. As a consequence floating point operations, e.g.
- store/load of floating point related registers during a fiber (context) switch
- are disabled.]
- [important TSX can not be used together with MSVC at this time!]
- Boost.Fiber uses TSX-enabled spinlocks to protect critical regions (see section
- [link tuning Tuning]).
- [endsect]
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