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Is your new GCC 4 rotten slow compared to GCC 3?
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Guenther Brunthaler
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PostPosted: Tue Sep 05, 2006 2:28 pm    Post subject: Is your new GCC 4 rotten slow compared to GCC 3? Reply with quote

Hi all,

After all I've heard, the new GCC4 generates better code than GCC3, but it also requires considerably more memory to work.

This can become a problem if you have a machine which has less RAM installed than GCC4 is using (some reports say it's using more than 2 GB for compiling a single C++ file in extreme cases).

In such situations, the reason why your machine is becoming so darn slow, is because of permanent swapping, also known as "thrashing".

In such cases, it is not uncommon for the total compilation speed to go down by a factor of 1000. (That is because the CPU(s) of your system can't do anything most of the time; they just wait for data from the hard disk nearly all the time.)

One possibility to fix the problem is of course to add more RAM.

But in many cases there is also an easy and cheap alternative to that:

If you were following the standard Gentoo setup guide when you installed your system, you might have the following entry in your /etc/make.conf:

MAKEOPTS="-j2"

Change this to

MAKEOPTS="-j1"

and your GCC will only need half the memory of before most of the time!

If you are lucky, then the amount of your installed RAM will again be enough to avoid the thrashing. And even if not, it will certainly thrash less.

How does it work?

The -j option in $MAKEOPTS specifies the number of parallel compilations that can take place at the same time.

-j2 thus means that 2 GCC4 processes will be compiling different source files when make has to compile a bunch of source files.

If you have memory enough, this is fine because both GCC's can share their code, and if one of the GCCs is waiting for more data from the disk, the other can continue compiling; thus minimizing unused processor power.

But if you have too little memory, the disk trashing which occurs will nullify any such advantage anyway, so it's better to only run a single GCC.

Note however, that decreasing the number of parallel compilations is probably not a good idea when using the distributed GCC package ("distcc"). It this case you must ensure somehow else that no more than one GCC4 process is run on a single machine (which too little RAM) by distcc.
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irondog
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PostPosted: Tue Sep 05, 2006 8:19 pm    Post subject: Reply with quote

Remember, that even when GCC4.1 isn't "trashing" it's much slower than GCC3.x.
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uweklosa
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PostPosted: Tue Sep 05, 2006 8:37 pm    Post subject: Reply with quote

I don't have this impression on my system.
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irondog
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PostPosted: Tue Sep 05, 2006 9:02 pm    Post subject: Reply with quote

Maybe much is overdone. I remember I benched gcc 3.x against 4.0 and 4.0 against 4.1. In both cases the newer versions were slower.
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zxy
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PostPosted: Wed Sep 06, 2006 10:01 pm    Post subject: Reply with quote

Does gcc use /var/tmp/portage/..... for temp files (if it uses temp files at all) or is there some other folder.

I usualy mount /var/tmp/portage to tmpfs before compiling, so disk usage is (usualy) only while decompressing tar files. But sometimes I can still see light go on during compile - it might still be ccache.
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Maedhros
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PostPosted: Thu Sep 07, 2006 9:59 am    Post subject: Reply with quote

zxy wrote:
Does gcc use /var/tmp/portage/..... for temp files (if it uses temp files at all) or is there some other folder.

Well, that's where portage builds the sources, so the object files are put there after being compiled. Wine regularly uses more than 1GB in /var/tmp/portage when compiling here though, so unless you have ridiculous amounts of ram you might be hitting swap anyway.
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agent_jdh
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PostPosted: Thu Sep 07, 2006 10:17 am    Post subject: Reply with quote

I've only got 512MB here and gcc-4.x appears quicker than the 3.x series. Just my 2p. Using makeopts=-j2 afaik.
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Guenther Brunthaler
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PostPosted: Thu Sep 07, 2006 2:11 pm    Post subject: Reply with quote

agent_jdh wrote:
I've only got 512MB here and gcc-4.x appears quicker than the 3.x series. Just my 2p. Using makeopts=-j2 afaik.


I would really like to use GCC4 because of its better code generator.

But then - you've also read those reports about the extreme hunger for RAM by GCC4, haven't you?

However, it certainly depends on the source file which is being compiled.

C source files won't need too much memory, no matter which compiler is used.

But C++ source files are a different story.

I remember watching extreme slow compilation progress when the QT libraries have been recompiled (although I was still using the 3.4.6 compiler).

Which means GNOME users might be happier than KDE users with GCC4.

So may I ask: Did you also emerge the QT libraries? (And watch them being compiled.)

If you didn't, it may be a good explanation for those conflicting reports ("faster" vs. "my swap file goes through the roof").
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syg00
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PostPosted: Thu Sep 07, 2006 2:21 pm    Post subject: Reply with quote

zxy wrote:
I usualy mount /var/tmp/portage to tmpfs before compiling, so disk usage is (usualy) only while decompressing tar files.
Mmmm - tmpfs uses page cache. Page cache is subject to swapping.

Anybody who is memory constrained and worried about the subject of this thread, may like to keep this in mind.
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agent_jdh
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PostPosted: Thu Sep 07, 2006 2:43 pm    Post subject: Reply with quote

Guenther Brunthaler wrote:
agent_jdh wrote:
I've only got 512MB here and gcc-4.x appears quicker than the 3.x series. Just my 2p. Using makeopts=-j2 afaik.


I would really like to use GCC4 because of its better code generator.

But then - you've also read those reports about the extreme hunger for RAM by GCC4, haven't you?

However, it certainly depends on the source file which is being compiled.

C source files won't need too much memory, no matter which compiler is used.

But C++ source files are a different story.

I remember watching extreme slow compilation progress when the QT libraries have been recompiled (although I was still using the 3.4.6 compiler).

Which means GNOME users might be happier than KDE users with GCC4.

So may I ask: Did you also emerge the QT libraries? (And watch them being compiled.)

If you didn't, it may be a good explanation for those conflicting reports ("faster" vs. "my swap file goes through the roof").


QT/KDE here. Certainly didn't notice any significant difference - it still takes ages to compile, just like it did with gcc-3.x! An emerge -e world is actually a bit faster now.
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Guenther Brunthaler
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PostPosted: Thu Sep 07, 2006 2:58 pm    Post subject: Reply with quote

agent_jdh wrote:
QT/KDE here. Certainly didn't notice any significant difference - it still takes ages to compile, just like it did


Fine!

Then I'll also dare it and recompile everything for GCC4 on occasion.

Except on that 128 MB notebook.
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devsk
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PostPosted: Thu Sep 07, 2006 6:54 pm    Post subject: Reply with quote

zxy wrote:
Does gcc use /var/tmp/portage/..... for temp files (if it uses temp files at all) or is there some other folder.
gcc doesn't create temp files if -pipe is used. So, that's one sure way of bypassing disk during compiles. Always have -pipe in your CFLAGS/CXXFLAGS. (only reason you would not want to have -pipe in your CFLAGS is if you are not using GNU assembler, which nobody on linux will do)
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