Re: [patch 2.6.13-rc3a] i386: inline restore_fpu

[Andrew Morton <akpm@osdl.org>]

Chuck Ebbert <76306.1226@compuserve.com> wrote:
>
> 
>   This patch makes restore_fpu() an inline.  When L1/L2 cache are saturated
> it makes a measurable difference.
> 
>   Results from profiling Volanomark follow.  Sample rate was 2000 samples/sec
> (HZ = 250, profile multiplier = 8) on a dual-processor Pentium II Xeon.
> 
> 
> Before:
> 
>  10680 restore_fpu                              333.7500
>   8351 device_not_available                     203.6829
>   3823 math_state_restore                        59.7344
>  -----
>  22854
> 
> 
> After:
> 
>  12534 math_state_restore                       130.5625
>   8354 device_not_available                     203.7561
>  -----
>  20888
> 
> 
> Patch is "obviously correct" and cuts 9% of the overhead.  Please apply.

hm.  What context switch rate is that thing doing?

Is the benchmark actually doing floating point stuff?

We do have the `used_math' optimisation in there which attempts to avoid
doing the FP save/restore if the app isn't actually using math.  But
<ancient recollections> there's code in glibc startup which always does a
bit of float, so that optimisation is always defeated.  There was some
discussion about periodically setting tasks back into !used_math state to
try to restore the optimisation for tasks which only do a little bit of FP,
but nothing actually got done.

> Next step should be to physically place math_state_restore() after
> device_not_available().  Would such a patch be accepted?  (Yes it
> would be ugly and require linker script changes.)

Depends on the benefit/ugly ratio ;)