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

[H. Peter Anvin]

Followup to:  <[e-mail]>
By author:    Chris Friesen <[e-mail]>
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
-- 
<[e-mail]> at work, <[e-mail]> in private!
"Unix gives you enough rope to shoot yourself in the foot."
Architectures needed: ia64 m68k mips64 ppc ppc64 s390 s390x sh v850 x86-64