Re: ufsstat - testers / feedback wanted!

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).

Eric


-- 
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Eric Anderson        Sr. Systems Administrator        Centaur Technology
Anything that works is better than anything that doesn't.
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