Re: msync() more expensive than fsync()?

["H. Peter Anvin" <hpa@zytor.com>]

Followup to:  <3E92FAE6.8000300@nortelnetworks.com>
By author:    Chris Friesen <cfriesen@nortelnetworks.com>
In newsgroup: linux.dev.kernel
> 
> 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.
> 

Because fsync() does less work.  In particular, fsync() doesn't do the
work you want: unlike msync() it doesn't guarantee that memory maps
are consistent with the filesystem.

	-hpa
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