HP OpenVMS Systems

Memory Interlocking Granularity? (take II)

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The Question is:

 
In a previous "Ask the Wizard" article (6984 & 7383) the OpenVMS Wizard stated:
 
"The interlocked instructions can typically lock ranges of memory rather larger
 than the target bit (or target queue entry), ... thus bitlocks located near
 each other in virtual memory have unexpected contention. This contention will
 not disrupt the corre
ct opreration of the bitlocks, but it can slow the access to the bitlocks."
 
You mention that the granularity of memory affected by a hardware interlock is
 implementation specific, but where can we find out what this is for various OS
 versions/hardware, especially OpenVMS v7.3-1 and ES40/45s hardware ?
 
 

The Answer is :

 
  There is no particular specification of this value, and the granularity
  of the memory interlock mechanism can be any value from an aligned
  longword to all of system physical memory on VAX and a quadword to 128
  bytes (recommended) to all of physical memory on Alpha -- this interlock
  granularity decision is otherwise left entirely to the particular platform
  design team(s) involved.
 
  The answer to your question is thus a generic 128 byte separation
  recommendation, and a further recommendation that you not code any
  further derivative assumptions into applications.
 
  For related (supporting) information, please see the Alpha LDx_L
  and STx_C mechanisms.  For additional related information, please
  see the 21264 directory on Freeware V4.0 and the SRM_CHECK tool.
 

  

     

     
answer written or last revised on ( 8-DEC-2002 )
     
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