Is your new GCC 4 rotten slow compared to GCC 3?

[Guenther Brunthaler]

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.

[irondog]

Remember, that even when GCC4.1 isn't "trashing" it's much slower than GCC3.x.
_________________
Alle dingen moeten onzin zijn.

[uweklosa]

I don't have this impression on my system.

[irondog]

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.
_________________
Alle dingen moeten onzin zijn.

[zxy]

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.
_________________
Nature does not hurry, yet everything is accomplished.
Lao Tzu

[Maedhros]

[agent_jdh]

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

[Guenther Brunthaler]

[syg00]

[agent_jdh]

[Guenther Brunthaler]

[devsk]