The content of this blog is my personal opinion only. Although I am an employee - currently of Imagination Technologies's MIPS group, in the past of other companies such as Intellectual Ventures, Intel, AMD, Motorola, and Gould - I reveal this only so that the reader may account for any possible bias I may have towards my employer's products. The statements I make here in no way represent my employer's position, nor am I authorized to speak on behalf of my employer. In fact, this posting may not even represent my personal opinion, since occasionally I play devil's advocate.

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Tuesday, March 06, 2012

malloc and transactyional memory (TM)


Dynamic memory allocation such as malloc is a classic example of why one may want memory accesses that can leave a transaxction.

Consider a simple mallocator, that allocates from a single queue or arena.
This might mean that multiple transactions, otherwise completely independent
might appear to interfere with each other through malloc.

(This interference might take several forms.
E.g. if the transactions are first performed totally independently, they may allocate the same memory.
E.g. or if the transactions malloc separate areas, but sequentially dependent, rollback of one might require rollback of the other.
(Although even this is an advanced TM topic, if the footprints overlap.))

Certainly, there can be malloc algorithms that minimize this - e.g. multiple areas, increasing the chance that different transactions might not interfere.

Or... the topic of this page:  permit malloc to exit the transaction
- so the [[parallel or concurrent]] mallocs may be properly synchronized, and receive independent memory addresses.

Q: what happens when a transaction aborts?
* Possibly could free the memory in the abort handler.  But this becomes less hardware, and more software TM.  Or at least a hybrid.
* Let garbage collection recover the memory speculative allocated within an aborted transaction.

(Q: what about [[deterministic finalization and aborted transactions]]?)

= Related Topics =

* [[speculative multithreading and memory allocation]]:
** [[malloc and speculative multithreading]]
** [[stack allocation and speculative multithreading]]

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