Myths
- "You don't need 64-bit software with less than 3 GB RAM"
- "There are less drivers for 64-bit OS"
- "You will need all new software, all 64-bit"
- "64-bit software is twice as fast"
AMD's 64-bit architecture extension
AMD64 introduces one new mode to the processor, the Long mode, consisting of the 64-bit mode and the Compatibility mode. The former is the new 64-bit environment, the latter a compatibility implementation to run 32-bit code in that 64-bit environment. The current operating mode is connected to the code segment. By using instructions that change code segments (like syscall) one can switch between both submodes.
Currently the paging algorithm (derived from PAE) limits the physical address to 52 bits, while the virtual address space is 48-bits wide. Even with far less physical RAM this proves to be helpful for memory mapping.
Another important part of the new architecture is the extended number of registers, both the general purpose registers as well as the SSE registers have been doubled, you can now use 16 of each in the new 64-bit mode.
Software support
Support for 64-bit in software required a new ABI to be introduced as well as extending GCC with the new architecture. As an essential part in that, up to six function parameters are now passed in registers. Linux kernel support for 64-bit started by splitting off a new architecture tree, x86-64. Since 2.6.24 this was merged with the i386 tree to one common code base (x86).
In addition a compatibility layer (aka compat layer) provides support for execution of 32-bit binaries on 64-bit processors.
32-bit compatibility
Compatibility to 32-bit code has been a crucial goal in the design. First of all it allows to run unmodified 32-bit code inside a 64-bit environment. Syscalls are used to switch between both worlds.
The processor zero-extends all 32-bit addresses to 64-bit. Applications use the lower 4 GB of the address space. However, physically that can be mapped anywhere. The kernel manages the compat layer for all applications, meaning it resolves bitness differences, structure layouts and invokes the right library version (/lib{32,64}/ld.so).
Speaking of libraries - each library has to be present twice, once for 32-bit and once for 64-bit, which also includes all dependent ones down to the lowest.
With the Linux compat layer it is even possible to run an entire and unmodified 32-bit Linux installation with a 64-bit kernel.
Benchmarks
First we picked some real world benchmarks for our 32-bit vs. 64-bit comparisons. Oggenc, Mencoder and Povray as well as some compilation tests. Furthermore micro benchmarks were used to show specific performance differences for syscalls and 64-bit arithmetics.
We set up three system configurations - a 32-bit installation, a 64-bit installation and a combination of 32-bit installation with 64-bit kernel to challenge the compat layer. All tests were performed on a dual-core AMD-K8(tm) processor with 1 GB RAM.
The tests showed that the penalty of using the compat layer instead of running your 32-bit application on a native 32-bit kernel is about 1-2 percent. So it is almost negligible.
64-bit took the lead in the media encoding tests. Our Povray and Mencoder benchmarks took about 5% less time in the 64-bit case, Oggenc even 25%. Just C-compilation tests showed a performance advantage of 5% to 8% for 32-bit versus 64-bit.
Native arithmetic performance (64-bit data types used in 64-bit software vs. 32-bit data types used in 32-bit) showed a gain of 10% for the 64-bit case. Using 64-bit data types on 32-bit and 64-bit in the arithmetic performance test showed that 64-bit is more than twice as fast as 32-bit.
Downsides of 64-bit
A 64-bit execution environment and 64-bit software surely have their downsides, too. First there is the larger memory footprint. Binaries get larger because of an increased pointer size and 64-bit operands. This leads to higher memory transfer load and therefore increases cache utilization.
Myths revisited
- "You don't need 64-bit software with less than 3 GB RAM"
- Performance improvement even on systems with less than 3 GB RAM
- "There are less drivers for 64-bit OS"
- Irrelevant to Linux, hail open source
- "You will need all new software, all 64 bit"
- 32-bit compat layer performs very well and is transparent
- "64-bit software is twice as fast"
- Rarely the fact, software is usually optimized for 32-bit
Conclusion
Use a 64-bit system and stick to the compat layer if you have the need of running certain 32-bit applications.
Andre Przywara, Andreas Herrmann, Peter Oruba
-------------------------
This response is provided for informational purposes only, is provided “AS IS” and does not obligate AMD to provide any of the services, technology, or programs described.
Edited: 03/26/2008 at 05:07 AM by peteroruba
