April 12, 2004
64-bit Computing Linux Style
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Jack Horgan - Contributing Editor

by Jack Horgan - Contributing Editor
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64-bit processing has been around since the mid nineties in the UNIX workstation arena (SUN's Ultra Sparc, HP's PA-RISC and IBM Power) and supported in applications by EDA vendors. In contrast 64-bit architecture is relatively new to the PC world and EDA vendor application support on PCs even more recent. One might well ask “What does 64 bit offer over 32 bit?” An n-bit processor can process an n-bit number in a single clock cycle. A 32-bit processor can handle a number up to 2^32 or roughly 4.3 billion. By comparison a 64-bit processor can handle a number up to 2^64 or roughly 18.4 quintillion (18,400,000,000,000,000,000). This difference in “dynamic range”
gives a significant
performance edge to the 64-bit architecture when dealing with large numbers which occur in any number of technical fields.

Integer arithmetic of very large numbers using 64-bit registers would execute considerably faster than in a 32-bit architecture. However, decimal operations are already performed in double precision using floating point registers in 32-bit computers.

More importantly is the significant impact of 64-bit architecture in terms of address space. A 32-bit process can access only 4.3 billion bits or 4.3 Gigabytes of memory. With 64-bit computing processors can access 16 extrabytes or 16 billion gigabytes of memory. Although 4 gigabytes sounds like a lot of memory, it must hold the operating systems, the application program and the application data. If the total exceeds this limit, some of the memory content will be swapped out to disk and possibly swapped back in at a later time. Memory access times are orders of magnitude faster than disk access times. In a severely constrained memory system, one can encounter a
“thrashing” effect.

In certain cases entire data bases or large files could be contained in the virtual address space and even in physical memory for immediate accessibility. Some applications do their modeling or simulation with large data arrays that could be retained in memory. 64-bit architecture supports scalability in terms of data base size.

64-bit processors date all the way back to 1993 with Digital Equipments Alpha Chip. Since then there have been several 64-bit processors targeted at high end computing rather than desktop applications. In May 2001 Intel introduced the Itanium 64-bit chip for personal computers which was not very successful for a number of reasons including the infamous floating point problem. In July 2002 Intel introduced its successor Itanium 2. In April 2003 AMD introduced AMD64 processor family including Opeteron and Athlon. The two vendors have marketed different approaches.

Intel targeted the Itanium at high end computing that had been the province of UNIX operating systems with a dedicated 64-bit only chip. The 64-bit architecture called IA64 was a departure from the existing x86 consequently it was incompatibility with existing 32-bit software. Intel did not view this as a problem because its target market was pure 64-bit applications. In 2002 Intel shipped 100,000 Itanium chips.

AMD took a less revolutionary approach and offered a dual purpose chip. AMD designed a backwardly compatible 64 bit extension to the x86 instruction set architecture called x86-64. The x86-64 also doubled the number of general purpose and SSE registers to 16. AMD supports a legacy mode that runs legacy (16- or 32-bit) operating systems, a compatibility mode executing existing 32-bit applications without recompilation under a 64-bit operating system and a 64-bit mode that requires an application to be compiled for 64-bit execution. The last two modes can be executing simultaneously. In compatibility mode 32-bit applications run at full speed in hardware without emulation.

The AMD approach leverages the large installed base of x86 hardware and makes it attractive to ISV's. With compatibility mode there is less porting time, testing time and performance tuning than retargeting to exploit 64-bit architecture. Of course the 32-bit applications will not run any faster and will still have a 4 gigabyte memory limitation.

During the keynote speech at the Intel Developers Forum in mid February Intel CEO Craig Barrett announced that new Xeons and future Prescott-based CPUs will support the same 64-bit software as AMD's existing Athlon 64 and Opteron chips. And like those AMD chips, the new Intel CPUs will continue to support 32-bit apps. Intel will phase in the chips over time. By mid-2004, Intel will issue next-generation, dual-processor-capable Xeon chips (Nacona) with 64-bit extensions aimed at server and workstation markets. By late-2004, Intel will ship 64-bit Pentium 4 chips (Prescott), aimed at the workstation and desktop markets. In early 2005, Intel will ship 4-processor-capable Xeon (Potomac)
versions. The company will continue to develop and market the Itanium family for high-end server needs. Barrett noted that Microsoft and various Linux vendors will support Intel's new architecture in the second half of 2004.

Benchmarks show superior performance and lower cost for Linux PCs versus UNIX based workstations. Leading EDA vendors have previously announced support for 64-bit personal computers running under Linux (
Red Hat Enterprise Linux Chosen for Electronic Design Automation). In January Cadence announced support for 64-bit Linux
Platforms to offer customers the increased capacity and higher performance critical for designing the largest and most complex SoC designs. Last November
Synopsys Announced Support for AMD64 Architecture. Last September Mentor Graphics announced
DFT Support for AMD Opteron Processors and AMD64. Also last September Magma announced porting of all products to 64-bit Opteron.

The EDA Consortium is the international association of companies that provide tools and services that enable engineers to create the world's electronic products. The organization publishes recommendation for operating system support over time. This OS Roadmap recommends Red Enterprise Linux 3 from April 2004 through May 2006 and Red Enterprise Linux 4 from May 2005 through TBD for support of 64-bit computing on both Itanium 2 and AMD64 architectures. Consult their website
www.eda.org/resources_roadmap.jsp for the support plans of its members.

In conversation with Karen Bartleson, Director of Interoperability at Synopsys, I learned that Synopsys takes an annual platform survey of their customer base. Last October the response to the survey question: “What percent of your EDA jobs require 64-bit addressing?”, none said less than 10%, 80% said that less than 45% required 64-bit addressing and 5% said more than 75% of jobs require 64-bit addressing. Karen anticipates that the use of 64-bit addressing will continue to increase. For the question “What is your number one EDA platform”, the same survey reported 70% UNIX and 30% Linux. The responders said that next year the answer would be Linux 58% and UNIX
30%. Karen also said
that large designs simply will not run in a 32-bit environment, which is why Synopsys has for some time supported 64-bit processing in both the UNIX and Linux worlds.

In conversation with Eric Filseth, Cadence VP Marketing Digital IC Implementation, he said that demand for 64-bit computing has been intense from those dealing with large designs. Large designs will not fit in a 32-bit environment. The performance of core computational processing if one is forced to go to disk is simply unacceptable. Given that Cadence announced support for 64-bit Linux only in January the vast majority of 64-bit users are on UNIX. On a percentage basis he guesstimated that 90% of Cadence seats are still 32-bit. He believes that customers will typical have a few 64-bit servers for executing the more complex jobs but use 32-bit platforms for the desktop. Eric feels
customers prefer to use 32-bit machine if the design will fit. There is a perception that it will run faster there. He also estimated that three quarters of users are on UNIX versus one quarter on Linux. But Linux usage is growing fast. There is a customer perception of superior performance and lower cost of ownership. Like many vendors Cadence ships software in a form that can be used in multiple environments making it difficult to know exactly what the customers are really using.

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-- Jack Horgan, EDACafe.com Contributing Editor.

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