Re: [Boost-bugs] [Boost C++ Libraries] #9019: Bug: mapped_region shouldn't create a new file mapping but reuse the one provided in file_mapping

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #9019: Bug: mapped_region shouldn't create a new file mapping but reuse the one provided in file_mapping
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2013-08-21 09:35:42

#9019: Bug: mapped_region shouldn't create a new file mapping but reuse the one
provided in file_mapping
-------------------------+-------------------------------------------------
  Reporter:              |      Owner:  igaztanaga
  michal.fronczyk@…      |     Status:  new
      Type:  Bugs        |  Component:  interprocess
 Milestone:  To Be       |   Severity:  Optimization
  Determined             |   Keywords:  interprocess mapped_region
   Version:  Boost       |  file_mapping
  1.54.0                 |
Resolution:              |
-------------------------+-------------------------------------------------

Comment (by igaztanaga):

 I think all file mappings share the same cached pages as coherency must be
 maintained:

 http://msdn.microsoft.com/en-

 us/library/windows/desktop/aa366763(v=vs.85).aspx

 "With one important exception, file views derived from any file mapping
 object that is backed by the same file are coherent or identical at a
 specific time. Coherency is guaranteed for views within a process and for
 views that are mapped by different processes."

 (The exception is network files)

 I don't think the issue is related to destroying the file mapping (in fact
 the file mapping is destroyed just after mapping the memory with
 MapViewOfFile), but because the mapped_region is unmapped
 (UnmapViewOfFile). The OS knows that no other thread is mapping those
 pages so it could think it's a good opportunity to release them (although
 if there is enough file cache, it should be still there). I'm not sure how
 this can be fixed as it's quite OS dependent.

 Mapping the whole file shouldn't be a problem, at least in windows, as
 physical memory committed when the page is read (although I guess Windows
 will prefetch some pages).

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/9019#comment:4>
Boost C++ Libraries <http://www.boost.org/>
Boost provides free peer-reviewed portable C++ source libraries.