Re: ufsstat - testers / feedback wanted!

Brian Candler wrote:
> 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)

Yes. On 32-bit it looks something like that:

         cltd
         movl    $1 %eax
         movl    $0, %edx
         addl    -16(%ebp), %eax
         adcl    -12(%ebp), %edx
         movl    %eax, -16(%ebp)
         movl    %edx, -12(%ebp)

> 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?

C compilers not that smart yet, AFAIK :-)