Re: [linux-fbdev] Re: readl() and friends and eieio on PPC

[Richard Henderson <rth@cygnus.com>]

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On Thu, Aug 12, 1999 at 03:00:46PM +1000, Paul Mackerras wrote:
> Show us the numbers?

Attached is a quick userland study wrt main memory.  How much
more accurate to a real device do I need to get to convice you
that the test is valid enough?

As I see it, testing against main memory should be the lower
bound of the numbers, since it's the quickest to respond.  A
real device will take longer to respond, so any enforced delays
(or failures to write-combine) will only exagerate the difference.

Anyway, the results (in cycles) from my 533MHz sx164 are:

10
10
10
10
10
223
94
94
94
94

So the cost of wmb for 8 store+wmb, versus 8 stores with one wmb,
is over 9:1.

> I don't believe the frame-buffer guys will actually
> see any measurable improvement in performance from taking out the
> eieio from readl/writel on PPC.

For grins, will you try the same test on your ppc?


r~

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#include <stdio.h>

main()
{
  int i;
  unsigned s, e;
  unsigned long mem;

  for (i = 0; i < 5; ++i)
    {
      asm("rpcc %0
	   stq $31,%2
	   stq $31,%2
	   stq $31,%2
	   stq $31,%2
	   stq $31,%2
	   stq $31,%2
	   stq $31,%2
	   wmb
	   stq $31,%2
	   rpcc %1"
	: "=r"(s), "=r"(e), "=m"(mem));
      printf("%u\n", e-s);
    }

  for (i = 0; i < 5; ++i)
    {
      asm("rpcc %0
	   stq $31,%2
	   wmb
	   stq $31,%2
	   wmb
	   stq $31,%2
	   wmb
	   stq $31,%2
	   wmb
	   stq $31,%2
	   wmb
	   stq $31,%2
	   wmb
	   stq $31,%2
	   wmb
	   stq $31,%2
	   wmb
	   rpcc %1"
	: "=r"(s), "=r"(e), "=m"(mem));
      printf("%u\n", e-s);
    }
}