[LC++]file copying

[Paul Gearon]

Jan Pfeifer wrote:
> i thought about this, but then in one test i did, i copied 10000 times a
> small file that would fit entirely in the buffer I allocated and it was
> still 1.5 times slower than /bin/cp ...
> 
> do you understand in this case why the difference ?

A couple of options come to mind, but I'll confess I'm surprised at the 
time difference.  Are you absolutely sure you're not seeing the cache 
come into effect here?  The first time you do any kind of copy then that 
part of the filesystem will be sitting in memory and all subsequent 
reads will be from RAM instead of the disk.  All writes (especially for 
small buffers) will return immediately and be scheduled for flushing to 
disk at a later time.  It's VERY hard to avoid these efficiencies for 
the purpose of benchmarking!

Assuming you've taken these things into account... what optimisation are 
you using?  The code that a compiler builds can make a significant 
difference to execution time.

Also, if you're comparing your code to cp for copying a small file then 
I'm guessing that you're executing the program once for each copy.  If 
that's the case then the executable file format may also be having an 
effect.  Is the executable stripped?  Is it ELF or a.out (probably ELF, 
but since you have a problem here it's worth asking)?  Are your 
functions inlined, etc?  Try lots of options here.

There are all kinds of things that could be making your code run slower 
than cp.  Normally you'd only worry about this if your code is very slow 
and you've profiled it to demonstrate that this is a particular concern 
for you.  However, if it's really bothering you then try playing with 
various optimisations and executable file formats.

If you want to isolate the file i/o differences between your code and cp 
then try comparing them when copying very large files.

-- 
Regards,
Paul Gearon

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