"Jeff Flinn" <TriumphSprint2000@hotmail.com> wrote in message news:c22in1$god$1@sea.gmane.org...
This topic has come up on the spirit list. Originally I was a proponent of mmap approach based on experiencing major performance improvements with earlier version of Windows particularly Windows CE. Today I've seen much less difference in performance. I've been told that Windows 2K/Xp using NTFS memorymaps files under the covers, and unless your actively adressing into the memory range there's little to be gained, atleast on Windows. Perhaps other os do this as well?
This is an issue that I have researched before. I have compared the performance of Win32 memory mapped file API, synchronous file I/O API along with iostreams and C-style buffered I/O (stdio.h). Still have the benchmark prog but it uses a few library classes of my own so if I'd try to post enough of the source here so it can be built and run, it'll be rather unwieldy. It used fairly large I/O buffers (16K) to maximize throughput and tested both sequential and random reading and writing performance. Actually, I even had a provision to test file creation as opposed to writing to an already allocated space since it might have put mapped files at a disadvantage. The summary of multiple runs is on the attached chart. The chart is in logarithmic scale so actual numbers should be irrelevant. To provide a reference point, the test was run on a Xeon 1.7G x 2 machine with 2G RDRAM under Win.2003 Server and allocated the file on an NTFS-formatted 7200 RPM IDE drive connected to a non-RAID (and, as far as I can tell, non-cached) onboard IDE adapter. Typical run times for a 256M test file are 0.3-0.5 sec using memory mapped files, 0.5-0.6 sec using file I/O (I'm not even gonna mention iostreams and stdio here, you'll see how atrocious they are from the chart). Short version - memory mapped files are SO much faster, it is not even funny. I did not go the extra mile and test the asynchronous I/O (scatter/gather reads and writes); my expectation is that they should beat memory mapping but not by much. 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