Diff for /site/data/goals/Attic/threads.cgi between versions 1.4 and 1.5

version 1.4, 2004/02/01 15:59:52 version 1.5, 2004/02/15 20:04:05
Line 62  create an abstraction for FAST software Line 62  create an abstraction for FAST software
 well, which will allow traditional hardclock() and statclock() distribution  well, which will allow traditional hardclock() and statclock() distribution
 to operate across all cpus.  to operate across all cpus.
 <P>  <P>
   <CENTER>The IPI Messaging Subsystem</CENTER>
   The LWKT model implements an asynchronous messaging system for communication
   between cpus.  Basically you simply make a call providing the target cpu with
   a function pointer and data argument which the target cpu executes 
   asynchronously.  Since this is an asynchronous model the caller does not wait
   for a synchronous completion, which greatly improves performance, and the 
   overhead on the target cpu is roughly equivalent to an interrupt.  
   IPI messages operate like FAST Interrupts... meaning that they preempt 
   whatever is running on the target cpu (subject to a critical section), run,
   and then whatever was running before resumes.  For this reason IPI functions
   are not allowed to block in any manner whatsoever.  IPI messages are used
   to do things like schedule threads and free memory belonging to other cpus.
   <CENTER>The IPI-based CPU Synchronization Subsystem</CENTER>
   The LWKT model implements a generalized, machine independant cpu
   synchronization API.  The API may be used to place target cpu(s) into a 
   known state while one is operating on a sensitive data structure.  This
   interface is primarily used to deal with MMU pagetable updates.  For
   example, it is not safe to check and clear the modify bit on a page table
   entry and then remove the page table entry, even if holding the proper lock,
   because a userland process running on another cpu may be accessing or
   modifying that page and create a race between the TLB writeback on the
   target cpu and your attempt to clear the page table entry.   The proper
   solution is to place all cpus that might be able to issue a writeback
   on the page table entry (meaning all cpus in the pmap's pm_active mask)
   into a known state first, then make the modification, then release the cpus
   with a request to invalidate their TLB.
   The API implemented by DragonFly is deadlock-free.  Multiple cpu
   synchronization activities are allowed to operate in parallel and this 
   includes any threads which are mastering a cpu synchronization event for
   the duration of mastering.  Even with this flexibility, since the cpu 
   synchronization interface operates in a controlled environment the call
   back functions tend to work just like the callback functions used in the
   IPI messaging subsystem.
 <CENTER>Token Passing Primitives Not Mutexes</CENTER>  <CENTER>Token Passing Primitives Not Mutexes</CENTER>
 <P>  <P>
 The LWKT model implements a token passing primitive rather then a mutex  The LWKT model implements a token passing primitive rather then a mutex

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  Added in v.1.5