HP OpenVMS Systems

pool management and fragmentation

» close window

The Question is:

 
I remember hearing something about VMS 7.0+ introducing something called
"memory fragmentation."  Supposedly, technically, with SRP, IRP, & LRPs going
away, the NPAGEDYN pool can become fragmented over time.  A reboot or some
third party tool were the recommended solutions.  Question:  Is any of this
true?  If so, do you have any additional info?  Thanks a lot!
 

The Answer is :

 
  The Wizard suspects there may be confusion here with a discussion of
  the OpenVMS VAX V6.0 and OpenVMS Alpha V1.0 releases, with the then-new
  adaptive pool management design was first shipped, and confusion over
  the meaning of some of the nonpaged pool information displayed by the
  SHOW MEMORY command.  (Code that allocated large blocks of pool and
  that then erroneously freed these blocks via multiple deallocations
  of smaller blocks, was found to have contributed to problems reported
  at that time, as well.)
 
  The design of the adaptive pool management was found to be sound (and
  the design continues to be used), and it was found to have significantly
  better performance than the older memory management design, and with
  memory requirements within +/- 5% of the older design.
 
  For some related information around a pool expansion problem that was
  corrected many years ago, please see the ECO kit(s) that superceed the
  VAXSYS01_061 kit.  (For information from the VAXSYS01_061 ECO.)
 
  These old ECOs are not relevent to V7.x vintage releases.
 
  As is the case with disk fragmentation, some level of pool fragmentation
  is entirely normal and expected behaviour.  The Wizard cannot reasonably
  recommend the use of any disk or pool defragmentation tool without knowing
  that the fragmentation is actually causing a visible performance problem.
  (And if it is, the Wizard would be interested in knowing more about the
  particular situation and application behaviour that has induced the
  fragmentation.)
 
  With large memory systems and OpenVMS Alpha V7.1, reclaimation can be
  disabled by setting the parameters NPAG_GENTLE and NPAG_AGGRESSIVE each
  to the value 100 -- this will cause the memory allocated to the lookaside
  lists to remain on the lists, and can reduce incidence of fragmentation.
  (There are cases where excessive pool fragmentation has been known to
  cause problems for some systems -- the systems where these cases have
  been seen typically involve large memory systems with large-scale locking
  activity.)
 
  With DECnet Phase IV on OpenVMS V7.1, there is a pool-related ECO kit
  available that should be installed: ALPSYSA_071.  This ECO allows
  larger lookaside list packet sizes, and is intended to better support
  typical DECnet Phase IV pool usage.
 

  

     

     
answer written or last revised on ( 29-JUL-1998 )
     
» close window