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Date:      Thu, 13 Jul 1995 03:15:10 -0700
From:      David Greenman <davidg>
To:        CVS-commiters, cvs-sys
Subject:   cvs commit: src/sys/vm default_pager.c default_pager.h
Message-ID:  <199507131015.DAA29449@freefall.cdrom.com>

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davidg      95/07/13 03:15:09

  Added:       sys/vm    default_pager.c default_pager.h
  Log:
  Oops, forgot to add the "default" pager files...
  
  NOTE: libkvm, w, ps, 'top', and any other utility which depends on struct
        proc or any VM system structure will have to be rebuilt!!!
  
  Much needed overhaul of the VM system. Included in this first round of
  changes:
  
  1) Improved pager interfaces: init, alloc, dealloc, getpages, putpages,
     haspage, and sync operations are supported. The haspage interface now
     provides information about clusterability. All pager routines now take
     struct vm_object's instead of "pagers".
  
  2) Improved data structures. In the previous paradigm, there is constant
     confusion caused by pagers being both a data structure ("allocate a
     pager") and a collection of routines. The idea of a pager structure has
     escentially been eliminated. Objects now have types, and this type is
     used to index the appropriate pager. In most cases, items in the pager
     structure were duplicated in the object data structure and thus were
     unnecessary. In the few cases that remained, a un_pager structure union
     was created in the object to contain these items.
  
  3) Because of the cleanup of #1 & #2, a lot of unnecessary layering can now
     be removed. For instance, vm_object_enter(), vm_object_lookup(),
     vm_object_remove(), and the associated object hash list were some of the
     things that were removed.
  
  4) simple_lock's removed. Discussion with several people reveals that the
     SMP locking primitives used in the VM system aren't likely the mechanism
     that we'll be adopting. Even if it were, the locking that was in the code
     was very inadequate and would have to be mostly re-done anyway. The
     locking in a uni-processor kernel was a no-op but went a long way toward
     making the code difficult to read and debug.
  
  5) Places that attempted to kludge-up the fact that we don't have kernel
     thread support have been fixed to reflect the reality that we are really
     dealing with processes, not threads. The VM system didn't have complete
     thread support, so the comments and mis-named routines were just wrong.
     We now use tsleep and wakeup directly in the lock routines, for instance.
  
  6) Where appropriate, the pagers have been improved, especially in the
     pager_alloc routines. Most of the pager_allocs have been rewritten and
     are now faster and easier to maintain.
  
  7) The pagedaemon pageout clustering algorithm has been rewritten and
     now tries harder to output an even number of pages before and after
     the requested page. This is sort of the reverse of the ideal pagein
     algorithm and should provide better overall performance.
  
  8) Unnecessary (incorrect) casts to caddr_t in calls to tsleep & wakeup
     have been removed. Some other unnecessary casts have also been removed.
  
  9) Some almost useless debugging code removed.
  
  10) Terminology of shadow objects vs. backing objects straightened out.
      The fact that the vm_object data structure escentially had this
      backwards really confused things. The use of "shadow" and "backing
      object" throughout the code is now internally consistent and correct
      in the Mach terminology.
  
  11) Several minor bug fixes, including one in the vm daemon that caused
      0 RSS objects to not get purged as intended.
  
  12) A "default pager" has now been created which cleans up the transition
      of objects to the "swap" type. The previous checks throughout the code
      for swp->pg_data != NULL were really ugly. This change also provides
      the rudiments for future backing of "anonymous" memory by something
      other than the swap pager (via the vnode pager, for example), and it
      allows the decision about which of these pagers to use to be made
      dynamically (although will need some additional decision code to do
      this, of course).
  
  13) (dyson) MAP_COPY has been deprecated and the corresponding "copy
      object" code has been removed. MAP_COPY was undocumented and non-
      standard. It was furthermore broken in several ways which caused its
      behavior to degrade to MAP_PRIVATE. Binaries that use MAP_COPY will
      continue to work correctly, but via the slightly different semantics
      of MAP_PRIVATE.
  
  14) (dyson) Sharing maps have been removed. It's marginal usefulness in a
      threads design can be worked around in other ways. Both #12 and #13
      were done to simplify the code and improve readability and maintain-
      ability. (As were most all of these changes)
  
  TODO:
  
  1) Rewrite most of the vnode pager to use VOP_GETPAGES/PUTPAGES. Doing
     this will reduce the vnode pager to a mere fraction of its current size.
  
  2) Rewrite vm_fault and the swap/vnode pagers to use the clustering
     information provided by the new haspage pager interface. This will
     substantially reduce the overhead by eliminating a large number of
     VOP_BMAP() calls. The VOP_BMAP() filesystem interface should be
     improved to provide both a "behind" and "ahead" indication of
     contiguousness.
  
  3) Implement the extended features of pager_haspage in swap_pager_haspage().
     It currently just says 0 pages ahead/behind.
  
  4) Re-implement the swap device (swstrategy) in a more elegant way, perhaps
     via a much more general mechanism that could also be used for disk
     striping of regular filesystems.
  
  5) Do something to improve the architecture of vm_object_collapse(). The
     fact that it makes calls into the swap pager and knows too much about
     how the swap pager operates really bothers me. It also doesn't allow
     for collapsing of non-swap pager objects ("unnamed" objects backed by
     other pagers).



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