Re: ufsstat - testers / feedback wanted!

Eric Anderson wrote:
> M. Warner Losh wrote:
> 
>> In message: <20051014091004.GC18513_at_uk.tiscali.com>
>>             Brian Candler <B.Candler_at_pobox.com> writes:
>> : On Thu, Oct 13, 2005 at 11:10:26AM -0700, Brooks Davis wrote:
>> : > > I don't think you can measure one single interger (or 64bit) 
>> increase in face : > > of a operation that has to access backing 
>> store.  Even if there is a : > > performance hit, you don't have to 
>> build your kernel with the option enabled.
>> : > : > The one thing I'd be worried about here is that 64bit updates are
>> : > expensive on 32bit machines if you want them to be atomic.  
>> Relative to
>> : > backing store they probably still don't matter, but the might be
>> : > noticable.
>> : : I'd be grateful if you could clarify that point for me. Are you 
>> saying that
>> : if I write
>> : :     long long foo;
>> :     ...
>> :     foo++;
>> : : then the C compiler generates code for 'foo++' which is not 
>> thread-safe?
>> : (And therefore I would have to protect it with a mutex or critical 
>> section)
>> : : Or are you saying that the C compiler inserts its own code around 
>> foo++ to
>> : turn it into a critical section, and therefore runs less efficiently 
>> than
>> : you'd expect?
>>
>> You have to protect this thread-unsafe operation yourself.
> 
> 
> For statistics gathering purposes though, should I worry about this, or 
> go for 'fast and imperfect' instead of 'perfect and slow'?  With 
> filesystems, I think it's more important to leave performance high and 
> get a notion of the statistics, rather than impact performance for 
> perfect stats (that you may only look at occasionally anyhow).

If you make it a #define macro then you can leave the choice for the compile
time.  Fast and lose when i++ and safe and slow when atomic_inc(&i, 1).

-- 
Andre