delay_tsc(): inefficient delay loop (2.6.16-mm1)

delay_tsc(): inefficient delay loop (2.6.16-mm1)

[Andreas Mohr]

Hello all,

I discovered that the delay loop used there (which simply found a new place
in 2.6.16-mm1; it existed much longer) is inefficient,
since it does an unnecessary subtraction *in the main loop* which also hits
asm code:

00000024 <delay_tsc>:
  24:   53                      push   %ebx
  25:   89 c3                   mov    %eax,%ebx
  27:   0f 31                   rdtsc
  29:   89 c1                   mov    %eax,%ecx
  2b:   f3 90                   pause
  2d:   0f 31                   rdtsc
  2f:   29 c8                   sub    %ecx,%eax
  31:   39 d8                   cmp    %ebx,%eax
  33:   72 f6                   jb     2b <delay_tsc+0x7>
  35:   5b                      pop    %ebx
  36:   c3                      ret

With such a patch:

--- linux-2.6.16-mm1/arch/i386/lib/delay.c.orig	2006-03-23 12:52:45.000000000 +0100
+++ linux-2.6.16-mm1/arch/i386/lib/delay.c	2006-03-24 12:53:01.000000000 +0100
@@ -44,10 +44,12 @@
 	unsigned long bclock, now;
 
 	rdtscl(bclock);
+	/* offset with bclock to have very simple comparison below */
+	loops += bclock;
 	do {
 		rep_nop();
 		rdtscl(now);
-	} while ((now-bclock) < loops);
+	} while (now < loops);
 }
 
 /*

the result is:

00000024 <delay_tsc>:
  24:   89 c1                   mov    %eax,%ecx
  26:   0f 31                   rdtsc
  28:   01 c1                   add    %eax,%ecx
  2a:   f3 90                   pause
  2c:   0f 31                   rdtsc
  2e:   39 c8                   cmp    %ecx,%eax
  30:   72 f8                   jb     2a <delay_tsc+0x6>
  32:   c3                      ret

Improvement: no unnecessary stuff after having hit the timer target value
(read: faster), better power saving, 4 bytes less opcodes.

The patch above could be considered weird since it fiddles with "loops"
directly. A possibly cleaner way would be to introduce a new variable
end = bclock + loops.

Comments? Anything that I'm missing here as to why it hasn't been done that
way before?

If this holds water then I'll submit a final patch soon.

Thanks!

Andreas Mohr

Re: delay_tsc(): inefficient delay loop (2.6.16-mm1)

[Ray Lee]

On 3/24/06, Andreas Mohr <andi@rhlx01.fht-esslingen.de> wrote:
> +       loops += bclock;
[...]
> -       } while ((now-bclock) < loops);
> +       } while (now < loops);

Erm, aren't you introducing an overflow problem here?

if loops is 2^32-1, bclock is 1, the old version would execute the
proper number of times, the new one will blow out in one tick.

Re: delay_tsc(): inefficient delay loop (2.6.16-mm1)

[Edgar Toernig]

Andreas Mohr wrote:
>
>  	rdtscl(bclock);
> +	/* offset with bclock to have very simple comparison below */
> +	loops += bclock;
>  	do {
>  		rep_nop();
>  		rdtscl(now);
> -	} while ((now-bclock) < loops);
> +	} while (now < loops);
>  }

Hehe, optimizing delay loops *g*  But your optimization is
wrong.  'loops+bclock' and/or 'now' is likely to wrap around
and then the test condition becomes bogus.

Ciao, ET.

Re: delay_tsc(): inefficient delay loop (2.6.16-mm1)

[Andreas Mohr]

Hi,

On Fri, Mar 24, 2006 at 09:22:51AM -0800, Ray Lee wrote:
> On 3/24/06, Andreas Mohr <andi@rhlx01.fht-esslingen.de> wrote:
> > +       loops += bclock;
> [...]
> > -       } while ((now-bclock) < loops);
> > +       } while (now < loops);
> 
> Erm, aren't you introducing an overflow problem here?
> 
> if loops is 2^32-1, bclock is 1, the old version would execute the
> proper number of times, the new one will blow out in one tick.

Doh. That's what happens if you get too excited about some new trick...
Back to the drawing board, methinks.

Andreas Mohr

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