Re: Memory mapped file pages getting cleared when memory overcommi
- From: Michael Vogt <MichaelVogt@xxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Sat, 27 Jan 2007 11:46:00 -0800
I don't think the file should even be really closed. Even though the
program dose a close of the file handle and the mapping object, it never did
the UnmapViewOfFile. Should not the system keep the file open because of
that.
"Alexander Grigoriev" wrote:
I guess, that the underlying backing file becomes deleted and inaccessible.
for page-out. Thus, any pageout discards the page. I don't like this
behavior, but it could be by design. If it is, it should better be
documented.
"Michael Vogt" <MichaelVogt@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:E14D5FFD-DF41-40D9-A694-05D1CB9010D0@xxxxxxxxxxxxxxxx
I have a win32 program running on XP Pro that is using a memory mapped
file
to create shared memory. I find that some or all pages of that memory are
being cleared in memory overcommit situations.
The program does not actually need the persistence of a file, so the
CreateFile() is called with flag FILE_FLAG_DELETE_ON_CLOSE. After doing
the
CreateFileMapping()and MapViewOfFile() the program does a CloseHandle() on
the file handle and on the handle to the file mapping object but still
keeps
"open" the pointer returned from MapViewOfFile().
Here is the relevant initialization code for the memory:
hFilePtr = CreateFile( TCPDRV32FILE,
GENERIC_READ | GENERIC_WRITE,
FILE_SHARE_READ | FILE_SHARE_WRITE,
NULL,
CREATE_ALWAYS,
FILE_ATTRIBUTE_HIDDEN |
FILE_ATTRIBUTE_TEMPORARY |
FILE_FLAG_DELETE_ON_CLOSE |
FILE_ATTRIBUTE_NORMAL,
NULL );
hFileMap = CreateFileMapping( (HANDLE)hFilePtr,
NULL,
PAGE_READWRITE,
0,
size,
TCPDRV32SHR );
lpMapAddr = MapViewOfFile( hFileMap,
FILE_MAP_ALL_ACCESS,
0, 0,
0 );
CloseHandle( hFileMap );
CloseHandle( hFilePtr );
In the relevant scenarios, no other application process opens the mapped
memory.
The memory is only getting lost/cleared when I do something like open 20
IE
sessions.
The memory clearing can be prevented by any of the following:
- Don't set FILE_FLAG_DELETE_ON_CLOSE
- don't close the file handle
- frequently read from the memory.
I need to know if this behaviour is expected because I need to rule out
that
the program is doing something else wrong.
The reason I suspect something else is going on is because in some
application scenarios, the problem does not occur. That is, there are
certain
application scenarios that experience the problem and others that do not.
In
at least some of those cases the memory is not being read any more
frequently
than a failing case.
See this post in the WinDbg newsgroup for some other background:
http://msdn.microsoft.com/newsgroups/default.aspx?dg=microsoft.public.windbg&tid=89af28e3-2d38-48c5-af7d-e17125ea88fb&m=1&p=1
Thanks for any help or insight.
Michael Vogt
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