[petsc-dev] Generality of VecScatter

[Jed Brown]

On Tue, Nov 29, 2011 at 10:35, Barry Smith <bsmith at mcs.anl.gov> wrote:

> Synchronization between threads frequently involves the use of spin-wait
> loops. These loops should make use of the PAUSE instruction to maximize
> performance and minimize power consumption. The PAUSE instruction can be
> added to application code now, as it is ignored on all known existing Intel
> architectures.
>

Where are going getting this from?


>
> So for Q all I need do is
>
> #define PAUSE
>
> and then use PAUSE in the spin-wait to get similar performance to the
> Intel?
>

http://software.intel.com/sites/products/documentation/hpc/compilerpro/en-us/cpp/lin/compiler_c/intref_cls/common/intref_sse2_pause.htm

http://software.intel.com/en-us/articles/introduction-to-hyper-threading-technology/

*Use the PAUSE Instruction to Optimize Code*
Intel recommends that software developers always use the PAUSE instruction
in spin-wait loops.

Starting with the Intel® Pentium® 4 processor, this recommendation was made
for the benefit of power savings. Executing a PAUSE in a spin-wait loop
forces the spin-wait to finish one iteration of the loop before starting
the next iteration. Halting the spin-wait loop reduces consumption of
non-productive resources - and thus power.

With a Hyper-Threading Technology supported processor, using the PAUSE
instruction optimizes processing of two threads. Pausing the spin-wait loop
frees more resources for the other processor, allowing faster completion of
its thread.

*Use OS Synchronization Techniques on Long Waits*
When a thread suspects a thread will take longer than an OS quantum of time
before a lock is released, use OS synchronization techniques to idle the
processor until the lock is released. Idling the Hyper-Threading Technology
supported processor frees the locked processor to use all resources
available to complete its execution. Use OS primitives to release the lock
when thread execution has completed.

If a thread suspects a thread will release a lock within an OS quantum, use
a spin-wait.
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