On Nov 29, 2011, at 10:41 AM, Jed Brown wrote: > 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?
The very first google hit for "pause instruction spin loop"; looks like they have the same web maintenance issues PETSc has :-) > > > 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.
