Linux 64 bit System Notes

• gcc under 64-bit linux is 64-bit

• Switches to optimise for Opteron: -march=k8 -malign-double -mfpmath=sse

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-- BillRice - 15 Feb 2006

Linux 64 bit System Notes

from LWN.net

http://lwn.net/Articles/79036/

The need for 32-bit libraries complicates system administration, however. An x86_64 Fedora system has many duplicated packages installed, and working with rpm can, occasionally, be a bit confusing. The rpm interface was not, perhaps, designed for dealing with a world where two packages have the same name and version number, but are still distinct. Unless you plan to leave the 32-bit world behind entirely, however, you will need two versions of the libraries. Chances are that most x86_64 systems will want to run 32-bit binaries for some time - in some cases, they perform better, and, in any case, some programs in FC2t2 (e.g. OpenOffice^?.org) are still built that way. Building applications can also be a bit of a challenge, at least a first. Quite a few makefiles and configure scripts assume that libraries live in /usr/lib. On a Fedora system, /usr/lib has the 32-bit versions of the libraries; the native versions live in /usr/lib64. A makefile which uses the default gcc (which compiles in 64-bit mode) and tries to explicitly link against things in /usr/lib will fail. Once you learn to recognize this problem, it gets easy to fix.

Your editor was naturally interested in performance issues. To that end, he built a version of bzip2 in both 64-bit and 32-bit mode and compared the results. Both compression and decompression ran about 10% faster in the 64-bit mode. With the x86_64 processor, better performance is generally expected in the native mode, mainly due to the additional registers which are available. The executable size and memory usage in 64-bit mode were larger, but not by much. A second test, using the SoundTouch^? library yielded a surprise, however: changing the tempo of a large sound file ran in less than 1/5 the time in 32-bit mode. The Athlon64 processor, it would seem, runs certain operations far more slowly in 64-bit mode; your editor has not, yet, had the time to track this one down.

From Newsforge

Performance and enhanced capabilities aside, the most valuable feature of the AMD64/EM64T is its ability to run 32-bit x86 binaries without a separate processor or operating system. This makes it much easier to slowly transition from a 32-bit to a 64-bit environment without having to change software applications. While AMD64 and EM64T processors are still very fast while in 32-bit mode, you won't be able to take advantage of any of the above-mentioned features and expanded resources (with the exception of the SSE and SSE2 instructions) if you're running a 32-bit operating system. Even if you're going to be running 32-bit binary programs, it pays to have a 64-bit operating system underneath them so that the rest of the system can run more efficiently. Fedora Core 3 (and the upcoming 4) is a good, safe bet for those who want to go 64-bit with as little cost and hassle as possible. Novell, Mandrake, and Red Hat have mostly 64-bit versions of their desktop, workstation, server, and corporate distributions, if that's the route you prefer. Gentoo Linux requires more time and tinkering to get things working properly in 64-bit mode, but for those of us who enjoy that sort of thing, that only makes Gentoo more appealing.

AMD Compilation Notes

• gcc under 64-bit linux is 64-bit
• most Makefiles default to /usr/lib, but 64-bit libraries are in /usr/lib64
• Use the -m32 switch in gcc to set compiler to 32-bit mode
• Switches to optimise for Opteron: -march=k8 -malign-double -mfpmath=sse
• Recommended to link with AMD MAth Core Library (AMDCL)

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-- BillRice - 15 Feb 2006

• amd-compiler-info.pdf: AMD Compiler Notes