Re: clearing filesystem cache for I/O benchmarks

[Hans Reiser]

Benjamin Rutt wrote:

> Hans Reiser <reiser@xxxxxxxxxxx> writes:
>
>  
>
> > when benchmarking, please be careful that you don't end up
> > benchmarking umount/mount, or sync, or..... it can be remarkably hard
> > to avoid such mistakes.....
> >    
>
> I agree, I've made some blunders like that in the past.  However for
> write tests, we are including fsync() time, once, at the end of a file
> write, since I feel it's unfair to trim that time.
fsync performance gives you different performance.  Better to write more 
stuff to flush the cache.

>  Not including
> fsync() time would only test the ability of the various parts of the
> I/O systems to do write buffering.  It's easy to do lots of write
> buffering, if you buy enough memory.  Forcing the disks to write is
> the only fair way to compare writes between I/O systems.
>  
It isn't fair.  fsync is a different code path, and may be less 
efficient.  Or more, depending on the fs.  reiser4 is currently not well 
optimized for fsync, maybe next year I will change that but not this 
week....

Benchmarking well is hard.....

Hans
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