Re: clearing filesystem cache for I/O benchmarks

[Hans Reiser <reiser@namesys.com>]

Andrew Morton wrote:

>(Please don't remove people from the email recipient list when doing kernel
>work.)
>
>Benjamin Rutt <rutt.4+news@osu.edu> wrote:
>  
>
>>Andrew Morton <akpm@osdl.org> writes:
>>
>>    
>>
>>>Benjamin Rutt <rutt.4+news@osu.edu> wrote:
>>>      
>>>
>>>> How can I purge all of the kernel's filesystem caches, so I can trust
>>>> that my I/O (read) requests I'm trying to benchmark bypass the kernel
>>>> filesystem cache?
>>>>        
>>>>
>>>Either delete the benchmark test files or
>>>      
>>>
>>I'm not sure I follow.  If I delete the benchmark files, I'll only
>>need to create them again later in order to do a read test, and I'll
>>have the same problem then, of how to eliminate the just-written-data
>>from cache.
>>    
>>
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 tend to try to use large enough filesets that small things like cache 
flush happenstance or bitmap loading overhead do not sway the benchmark.

Rebooting tends to work for resetting the OS thoroughly, though I would 
be curious to hear comments on whether one ought to power down the disk 
drive so that its cache flushes......;-)