RE: Updated Windows mmap() wrapper

On Mon, 19 Jan 2004, Bill Radcliffe wrote:

> I have often found the source code to these kinds of wrappers quite
> instructive. It should at least inform you on how they deal with some of the
> legacy issues discussed below. It may very well yield secrets that will help
> you. Maybe not, but it is certainly worth a shot IMHO.

The Cygwin mmap wrapper may be instructive, but the MinGW "wrapper" is
just a C prototype for Windows API functions, defines, and the
equivalent of a .lib file for linking with the DLL.

It will be a while before we explore using mmap with base offsets
other than zero.  It would require a re-write of cache.c.  The real
challenge with with cache views since if mmapped regions overlap, the
address of the conflicting pages is determined by the last mmap.  If
cache views overlap then serious problems can result since part of the
address space of the loser (the first request) will become invalid.
A possible solution is to reject any request (i.e. return NULL) for an
address region which overlaps with the address region used by a
different view.  A bitmap could be used to record which pages are
currently mapped, or the info could be maintained as a list of
ranges associated with the views.

Testing under Solaris shows that the overhead of the mmap call is
fairly small. The following are some timings to update 2GB of file
data (using default page flushing) with various mmap lengths:

One huge mmap (GraphicsMagick current approach):
real     2:25.228
user        4.187
sys        43.900

mmap per 16 MMU pages:
real     2:37.438
user        4.701
sys        47.228

mmap per MMU page:
real     3:29.311
user        9.552
sys      1:05.738

I think that the MMU page size is 8K for this system.

It remains to be seen how the overhead of memory mapping on other
systems compares.

Bob
======================================
Bob Friesenhahn
[email protected]
http://www.simplesystems.org/users/bfriesen



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