Re: [sheflug] CFLAG Question

["Wonkey Donkey" <wonkey_donkey [at] xxxxxxxxxxxxxxxx>]

----- Original Message ----- 
From: "James Rogers" <phb01jdr [at] shef.ac.uk>
To: "sheflug" <sheflug [at] sheflug.co.uk>
Sent: Tuesday, February 21, 2006 10:13 PM
Subject: Re: [sheflug] CFLAG Question


> Wonkey Donkey wrote:
>> Ok, first off, the distro is Gentoo, and the machine has a P4 3.2 
>> Northwood
>> CPU and 2Gb of very low latency ram.
>>
>> The machine is currently building the toolchain for Gentoo, and is using 
>> a
>> quite normal set of CLAGS, except for the fact that -O2 is now -O3. (I'm
>> running this purely as a test, its not a production machine or anything).
>> Everything I have read about it seems to suggest that the higher the -O
>> value, the better optimised the resulting code will be, but at the 
>> expense
>> of compilation time and memory usage.
>>
>> Compared with my usual -O2 setting, this thing is taking an absolute age 
>> to
>> compile and build. I did expect extra time being needed of course, but 
>> I'm
>> wondering just how beneficial the end result may be and if it is worth
>> bothering with -O3.
>>
>> As a general question, am I likely to see a significant speed improvement
>> once the machine is built and rebooted ? I don't mind the extra time if 
>> its
>> going to be worthwhile; but to give you an idea of the differences I'm
>> seeing right now, GCC usually takes around 25 minutes, whereas with -O3 
>> it
>> took almost an hour. Glibc is currently running and has taken well in 
>> excess
>> of an hour already; it usually takes around 40 minutes.
>>
>> Is a system built with -O3 going to be so much more improved than 
>> with -O2,
>> and if not, can anybody suggest in simple terms why there is such a
>> difference in compile times for such a small difference in the end result 
>> ?
>>
>> Thanks.
>>
>> Steve.
>>
>
> IMHO -O3 isn't worth it, for most apps. Beyond -O2 it's only really
> worth it for maths/physics simulations etc where the bottleneck is the 
> CPU.
> Personally i run -Os which is -O2 but then optimized for smaller binary
> size and is thought of highly on gentoo.org forums.
> When I last did some simulations there was a noticeable difference in
> runtime for 02 -> 03 but that was mainly floating point. Can't ever say
> I've seen much difference in normal system usage.
> Setting your USE flags minimally and tuning your filesystem give more
> noticeable improvement than O2->O3
>
> James
>

Thanks James. I'm going to let it run its course now anyway, just to see 
what its like with -O3.

One other thought that crossed my mind is this: is there any problem 
changing between -O2 and -O3 during the build of the complete system ?

Such as now, its bootstrapping and rebuilding the toolchain using -O3, would 
it be a major problem switching to -O2 to build the 'system'.... Just a 
thought.

As to the filesystem, yes, I was looking through the forums on that earlier 
too. The general opinion was that ext3 is still definitely the recommended 
way to go, which Im happy with as Ive always used it and never had any 
problems.

There are also a couple of interesting posts about which journalling method 
should be used, and the various associated flags. And also a very good post 
about fragmentation, optimisation etc, something I have never actually 
considered until now.

As for USE flags, mine are anything but minimal to be honest. But I'll be 
posting more on than subject in  the next day or two.

Steve. 


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