[Firebird-devel] FYI Hyperthreading - Threads can be locked out by incorrect spin-wait implementation

[Wayne S. <body1233_@_yahoo>]

http://www.intel.com/cd/ids/developer/asmo-na/eng/196575.htm?page=5

Excerpts:

"With Hyper-Threading Technology, spin-wait loops can delay the 
completion of one thread when another thread holds a lock, because the 
looping thread is repeatedly scheduled and executed, consuming vital 
shared resources in its loop."

"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."

"Your code should use the OS SYNCHRONIZATION techniques when the lock is 
suspected to be longer than an OS quantum. Use a spin-wait PAUSE command 
to wait for a lock when it's suspected to last less than an OS quantum."