msync() more expensive than fsync()?

[Chris Friesen]

I have some code that runs on a ramdisk-based filesystem.  Through a special 
device it mmaps a section of memory that is persistant over reboots, to be used 
for logging.  In order to guarantee that the logs were flushed to the memory 
area, I  assume that I need to use some kind of sync operation.  This is where 
things get a bit odd.

I did some testing with relatively small messages, 50 bytes or so, with results 
as follows:

Without any explicit flushing it takes 8 usec to log a message.

If I msync() only the pages that were touched in writing (usually 3 pages) it 
takes 39 usecs to log a message.

If I fsync() the entire file (200KB) it takes 12 usec to log a message.

Why the additional cost for msync()?  I would have thought it would be faster 
since it is explicitely for mmapped memory areas.  As a side note, the 
difference is even more extreme if a file is used on a disk-backed filesystem.

The kernel was 2.4.18.

Chris

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