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STMX is an actively maintained, high-performance concurrency library providing Software Transactional Memory for Common Lisp.

Home page and downloads:

Main features

  • Extremely intuitive to use and to write correct, thread-safe concurrent code.
  • Brings database-style transactions to Common Lisp by introducing transactional memory.
  • High performance implementation, benchmarked to reach up to 6 millions transactions per second per CPU core on commodity PC hardware.
  • Removes the need for traditional locks, mutexes and conditions - writing correct concurrent code with them is well known to be hard.
  • Transactional code is intrinsically deadlock-free: if two transactions conflict one of them will be re-executed.
  • Automatic commit and rollback: if a transaction completes normally it will be committed, if it exits with a non-local control transfer (signals an error, throws, or calls (go ...) to exit an atomic block) it will be rolled back.
  • Transactions are composable: they can be executed in a larger transaction, either in sequence (all-or-nothing) or as alternatives (try them in order until one succeeds).
  • Guarantees a consistent view of memory during transactions: concurrent updates from other threads are not visible - if the consistency cannot be guaranteed, the transaction will be automatically rolled back and re-executed from scratch.
  • Offers freedom of choice between blocking and non-blocking transactional functions: given either behaviour, it is trivial to transform it into the other.
  • Features transactional versions of popular data structures: hash tables, red-black trees, stack, fifo, etc.
  • Includes transactional data structure for multicast publish/subscribe
  • Creating new transactional data structures is easy.
  • Extensive test suite.
  • Tested on SBCL, CMUCL, CCL and ABCL.
  • Very simple to install with Quicklisp.

A quick-start guide and installation instructions are provided in the file

License: LLGPL

What STMX is NOT

In order not to confuse programmers - less experienced ones in particular - and to avoid rising unrealistic hopes, the author states the following about STMX:

  • it is not a quick hack to automagically transform existing, slow, single-threaded programs into fast, concurrent ones. No matter how much transactions can help, writing concurrent code still requires careful design and implementation - and testing. And refactoring takes time too.
  • it is not for optimization-focused programmers trying to squeeze the last cycle from their Common Lisp programs. STMX records an in-memory transaction log containing all reads and writes from/to transactional memory, then later (during commit) validates the transaction log against the latest data present in transactional memory and finally copies the transaction log onto the transactional memory while holding locks. STMX is quite optimized, but this machinery comes at an obvious performance cost with respect to hand-made, highly optimized locking code (but a good cross-check is to ask yourself how many people have the skill and patience to write such locking code without bugs).
  • it is not supposed to be used for all data structures in a Common Lisp program. STMX is intended only for the data accessed concurrently by multiple threads while being destructively modified by at least one thread. And even in that case, transactional memory is not always needed: for simple modifications, locking code is usually feasible; for complex structural modifications, STMX can help greatly.
  • it is not a serialization or persistence framework. Rather, messing with metaclasses and playing (allowed) tricks with slots contents as STMX does, quite likely does not mix well with serialization or persistence libraries such as CL-STORE or CL-MARSHAL, because they typically need full control on the slots of the objects to be serialized and de-serialized.
  • it is not a million dollar library from some deep-pocket company. At the moment, it is the work of a single person.


STMX is based on the concepts described in Composable Memory Transactions with the addition of a global version clock as described in Transactional Locking II

In particular:

  • transactional memory reads and writes are stored in a transaction log, and written values are copied into the actual memory location only during commit

  • each transactional memory location is locked only during commit, not while accessing it

  • conflicts, i.e. multiple transactions trying to write simultaneously the same memory location, are detected automatically during commit. In such case, one transaction will commit and all other ones will be rolled back and re-executed from the beginning

  • thanks to the global version clock, it cannot happen that a transaction sees an inconsistent view of transactional memory even if other threads modify it.

    The worst that can happen is an automatic rollback and re-execution of a transaction immediately before it can see an inconsistent view of transactional memory.