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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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Wednesday, February 09, 2011

Non-speculative or less speculative versus more speculative

http://semipublic.comp-arch.net/wiki/Non-speculative_or_less_speculative_versus_more_speculative

When discussing [[speculative execution]] techniques such as [[speculative multithreading]] (or [[skipahead]], or [[slipahead]], or ... )
we often need to talk about less speculative and more speculative threads or instructions.

E.g. a later, younger, more speculative load instruction may be blocked by an earlier, older, less speculative store instruction to the same address.

E.g. in [[SpMT]] or [[SkMT]] a less speculative thread may fork a more speculative thread.

Often in early work we only talk about non-speculative threads forking,
and do not discuss the possibility of a speculative thread forking.
Late, it is realized that it is entiirely reasonable for speculative threads to fork.
Thus, often we need to read "non-speculative" in early work,
and implicitly substitute "less speculative".

E.g. a common [[speculative thread management policy]] is to always keep the least speculativethreads,
and cancel any more speculativethreads,
when there is an opportunity to spawn a less speculative thread.

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Note that "less speculative" is not necessarily the same as "older".
E.g. in a [[fork-on-call]] thread, certain instructions may beindependent of the skipped code,
and be guaranteed to be executed;
whereas other instructions, inside a skipped function,
or older in the [[Von Neumann instruction sequence]],
but are actually more speculative, as in less likely to be executed.
However, it is often too hard to track this, so often discussions
will implicitly assume "less speculative" is "older".

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Note that it is not always possible to determine the order of speculative threads or instructions.
E.g. one may fork loop bodies out of order, with no known order,
and later string them together as memory items iterated on by the loop are encountered.
However, arbitrarily imposing an order simplifies many speculative algorithms,
even at the cost of potential performance.

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* See[[spawning versus forking a thread]].

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