linux-kernel - Re: [RFT] Port 0x80 I/O speed

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Date:	Wed, 12 Dec 2007 13:02:39 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Rene Herman <rene.herman@...il.com>
CC:	Alejandro Riveira Fernández 
	<ariveira@...il.com>, Linux Kernel <linux-kernel@...r.kernel.org>,
	dpreed@...d.com, Alan Cox <alan@...rguk.ukuu.org.uk>, pavel@....cz,
	andi@...stfloor.org, rol@...917.net,
	Krzysztof Halasa <khc@...waw.pl>, david@...idnewall.com,
	john@...ffel.org, linux-os@...logic.com
Subject: Re: [RFT] Port 0x80 I/O speed

Rene Herman wrote:
> On 12-12-07 01:09, Alejandro Riveira Fernández wrote:
> 
>>>>  On my AMD 3800 X2 (2000MHz) ULi M1697 2.6.24-rc5 i get:
>>>>
>>>>  cycles: out 1844674407370808, in 1844674407369087
>>>>  
>>>> It is not constant but variations are not significant afaics
>>> Eh, oh, I guess you need to compile as a 32-bit binary...
>>  
>>  I tried without -O2 as Nigel Cunningham...
>>
>>  cycles: out 1562, in 865
>>  cycles: out 1562, in 866
>>  cycles: out 1555, in 858
>>  cycles: out 1562, in 866
>>  
>>  With -m32 -O2
>>  cycles: out 1566, in 876
>>  cycles: out 1555, in 865
>>  cycles: out 1594, in 931
>>  cycles: out 1559, in 874
> 
> Great, thanks much for reporting. Sort of interesting in itself that 
> without -O2 you do still get correct results on 64-bit but for some 
> other time.
> 
> You're the first one to go significantly below 1 us it seems.

Make sure the CPU is actually running at full frequency.

It probably would have been better to have used gettimeofday() around a 
sufficiently big loop, so that we would have gotten wall time rather 
than cycles.

	-hpa
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