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Date: Mon, 1 Nov 2004 17:22:27 -0500 (EST)
From: linux-os <linux-os@chaos.analogic.com>
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To: dean gaudet <dean-list-linux-kernel@arctic.org>
cc: Linus Torvalds <torvalds@osdl.org>, Andreas Steinmetz <ast@domdv.de>,
       Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Richard Henderson <rth@redhat.com>, Andi Kleen <ak@muc.de>,
       Andrew Morton <akpm@osdl.org>, Jan Hubicka <jh@suse.cz>
Subject: Re: Semaphore assembly-code bug
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On Mon, 1 Nov 2004, dean gaudet wrote:

> On Mon, 1 Nov 2004, linux-os wrote:
>
>> On Mon, 1 Nov 2004, dean gaudet wrote:
>>
>>> On Sun, 31 Oct 2004, linux-os wrote:
>>>
>>>> Timer overhead = 88 CPU clocks
>>>> push 3, pop 3 = 12 CPU clocks
>>>> push 3, pop 2 = 12 CPU clocks
>>>> push 3, pop 1 = 12 CPU clocks
>>>> push 3, pop none using ADD = 8 CPU clocks
>>>> push 3, pop none using LEA = 8 CPU clocks
>>>> push 3, pop into same register = 12 CPU clocks
>>>
>>> your microbenchmark makes assumptions about rdtsc which haven't been valid
>>> since the days of the 486.  rdtsc has serializing aspects and overhead that
>>> you can't just eliminate by running it in a tight loop and subtracting out
>>> that "overhead".
>>>
>>
>> Wrong.
>
> if you were correct then i should be able to measure 1 cycle differences
> in sequences such as the following:
[SNIPPED...]

Who said? The resolution isn't even specified. Experimental
results with several different processors seem to show that
the resolution is about 4 cycles.

Script started on Mon 01 Nov 2004 04:48:04 PM EST
# ./tester
Timer overhead = 88 CPU clocks


1 nop = 4 CPU clocks
2 nops = 4 CPU clocks
3 nops = 4 CPU clocks
4 nops = 8 CPU clocks
5 nops = 8 CPU clocks
6 nops = 8 CPU clocks
7 nops = 8 CPU clocks
8 nops = 12 CPU clocks
# exit
Script done on Mon 01 Nov 2004 04:48:34 PM EST

Assembly :


nop8:	nop
nop7:	nop
nop6:	nop
nop5:	nop
nop4:	nop
nop3:	nop
nop2:	nop
nop1:	nop
 	ret

.global	nop1
.global	nop2
.global	nop3
.global	nop4
.global	nop5
.global	nop6
.global	nop7
.global	nop8


Cheers,
Dick Johnson
Penguin : Linux version 2.6.9 on an i686 machine (5537.79 BogoMips).
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