Re: [PATCH] Athlon/Opteron Prefetch Fix for 2.6.0test5 + numbers

[Nick Piggin]

Andrew Morton wrote:

> Andi Kleen <ak@xxxxxxx> wrote:
>
> > This is much more efficient than the previous workaround used in the kernel,
> > which checked for AMD CPUs in every prefetch(). This can be seen 
> > in the size of the vmlinux:
>
> That is hardly a serious comparison: the workaround is just to stop the
> oopses while this gets sorted out.  It makes no pretense at either
> efficiency or permanence.
>
>
> > Without patch:
> >   text    data     bss     dec     hex filename
> > 4020232  665956  169092 4855280  4a15f0 vmlinux
> > With patch:
> > 4011578  665973  169092 4846643  49f433
>
> hrm.  Why did data grow?
>
>
> > With prefetch check:    3.7268 microseconds
> > Without prefetch check: 3.65945 microseconds
>
> We don't know how much of this difference is due to removing the branch and
> how much is due to reenabling prefetch.
>
> It would be interesting to see comparative benchmarking between prefetch
> and no prefetch at all, see whether this feature is worth its icache
> footprint.

The test was on a pentium 4, so its only removing the extra code.

I think Andi's patch is required (especially because it fixes
userspace), and under the current cpu selection scheme, it is
implemented correctly (although I am now at a loss as to what the
generic thing is for).

The conditional compilation thing is a seperate issue. This patch may
have just broken a few camels' backs.

What is intriguing to me is the "Its only a 2% slowdown of the page
fault for every cpu other than K[78] for this single workaround. There
is no point to conditional compilation" attitude some people have.
Of course, its only 2% on a pagefault, not anywhere near 2% of kernel
performance as a whole, so maybe that is justified.

Just repeating though, that is a seperate issue and I think Andi's patch
is needed.


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