AIPS and Linux: A Historical Reminiscence

AIPS is a package to support the reduction and analysis of data taken with radio telescopes. It is most useful for arrays of telescopes like the VLA and VLBA. In the past few years, it has also been used successfully for “Space VLBI” (very long baseline interferometry) in conjunction with a small telescope on a Japanese satellite (HALCA or VSOP).

Figure 4. Screen Shot of a Typical AIPS Session

AIPS is what most of us would now describe as “legacy software”, having been originally coded in a truly ancient dialect of FORTRAN (predating even the venerable FORTRAN IV). AIPS now uses FORTRAN 77, although it has been digested successfully by at least one FORTRAN 90 compiler.

A Modcomp computer in Charlottesville was the first system to actually host a working AIPS system, and it quickly spread to a guest UNIX system hosted on an IBM 360 mainframe. From there, it spread in the early 1980s to VAX/VMS systems, often with an attached floating-point systems array processor (this peculiar device was the moral equivalent of the 80387 floating-point accelerators that some old-timers may remember being part and parcel of many 386 systems). In the late 1980s, UNIX came back into AIPS' universe in a big way, first with the Sun-3 series of Motorola-68020-based systems and then with a series of others, including Cray (Unicos), Convex and Alliant systems.

By the time the 80s were winding down, the dominance of VMS in the AIPS universe was being seriously questioned. Performance on new upstarts like Sun was starting to challenge their price/performance ratio, and the first SPARCstations totally blew them away. In the early 1990s, AIPS moved to a smorgasbord of UNIX variants: AIX, Stardent (briefly), Ultrix, HP-UX, SGI's Irix and DEC (oops, Compaq) OSF1. A port to an IBM 3090 was attempted, but failed due to accuracy problems with the non-IEEE floating-point format thereon. In the middle of this flurry of activity, the port to Linux by Jeff Uphoff was made.

In the fall of 1993, NRAO got a query from a radio astronomer at Virginia Tech in Blacksburg, requesting permission for one of his students to copy AIPS to his PC for an attempted port to a new system called Linux. (At that time, AIPS was still proprietary code, released to non-profit organizations under a rather cumbersome license and user agreement; this changed later.) Polite skepticism was the immediate reaction of most people then in the NRAO AIPS group, but we thought it would be an interesting exercise. We had been giving some thought to the issue of running AIPS on personal computers at the time, but had not pursued it.

A scant two weeks later, I received a technical support call from the same Jeff Uphoff, where he alluded to some difficulty in compiling one routine using f2c/gcc (remember, this was 1993, before g77 came to prominence). As it turned out, the problem was minor, and Jeff had successfully completed the whole port, even to the point of running the benchmark “Dirty Dozen Tasks” or DDT suite. This benchmark takes about an hour to run on a legacy SPARCstation 1 or SPARC IPX. On Jeff's poor little “uppieland” 386, it took over a day, but it ran to completion with acceptable accuracy results. We were equally impressed with his ability to print PostScript output on his little DeskJet 500 printer; our HP and QMS printers at the time all cost many thousands of dollars.

Needless to say, this feat and the contribution of the modifications necessary for Linux made a big impact at NRAO. Within a month, we had invited Jeff to visit and install a copy of Linux with AIPS running on it on a system in our lab. That was the dark and stormy evening referenced in the introduction above, and it saw both of us huddling over a (then state-of-the-art) Gateway 486/66 system, boot floppies in hand, busily preparing it for AIPS. Within a couple of hours, we had the system installed, looking for all intents and purposes like a Sun, even down to the Open Look window manager, and busily munching away on AIPS data. We benchmarked it at about half the speed of the Sparc IPX systems we had at the time. Seeing that the 486 cost a lot less than an IPX, this got our attention once more.

Within a few months of the original port, NRAO had Jeff Uphoff on its payroll, and the race was on to improve the performance of AIPS on Intel hardware. In the process, the NRAO Charlottesville Computing Division ended up with many Linux machines performing server duties, and several programmers and scientists volunteered for (in some cases, demanded) Linux desktop systems. The operating system was also spreading like wildfire on many of our home systems as an adjunct to, or in some cases a replacement for, that OS from Redmond.

However, it took the use of the EGCS version of the GNU g77 FORTRAN compiler to push the Intel/Linux platform to the forefront of the Radio Astronomy community. In 1995, using EGCS version 1.0.2, we (well, okay, it was Jeff again; why isn't he writing this article?) succeeded in getting AIPS to build under g77. This improved the AIPSMark (our benchmark, defined as 4000 divided by the elapsed time in seconds to run the DDT on our large dataset; bigger AIPSMarks are better and a Sparc IPX is 1.0) on a Pentium Pro 200 from 3.3 to about 5. With further coaxing via aggressive use of optimization parameters, the resulting AIPSMark went over 6. In this fell swoop, the price/performance curve that was previously occupied by Sun, IBM, DEC and HP was shattered once and for all. By 1998, NRAO was ordering Linux/Intel desktops as the workstation of preference for the scientist in place of SPARC Ultra systems. In 1999, Linux started to edge out the high-performance public workstations such as Alpha and high-end SPARC; our star performer in Charlottesville is a Pentium III Xeon 550MHz system with an AIPSMark of over 21. This is still not close to the stratospheric performance of top-of-the-line HP and Alpha systems (both around 34 AIPSMarks), but in price/performance terms, it's still a knockout.

During this time, another significant change came about. All this exposure to copylefted code was taking its toll on us. As mentioned earlier, AIPS was originally released under a restrictive user agreement that prohibited redistribution and was unpalatable or even unacceptable to some in our own astronomical community. Not only that, but the administrative costs associated with it were a burden.

Thus, several of us started a rather vigorous campaign to shift AIPS over to the GNU (Free Software Foundation's) General Public License on its next periodic release. There was virtually no opposition to the principle, and after a brief review by our business division, we were given the green light. Applying the three-paragraph copyleft statement to tens of thousands of files was no easy feat, but we managed to automate it. (Shell scripts and Perl scripts can do almost anything.) The distribution of AIPS took off at this point, and nowadays if you encounter a radio astronomer anywhere in the world, chances are she will have an AIPS CD-ROM somewhere or have it installed on her system.