Real-time responses for Linux possible, but unwanted

MontaVista Software has begun work to endow Linux with a feature currently possessed only by proprietary rivals: a guaranteed fast response time useful in everything from automotive control computers to video players.

The Californian start-up focuses on embedded computing devices such as DVD players and telecommunications equipment. But the company hasn't been able to bring Linux to a significant part of the embedded market, devices requiring a "hard real time" operating system that can assure a response within a specific and short span of time.

MontaVista plans to announce the hard real-time move on Tuesday. But it issued a prototype patch on Friday that would add the ability.

Hard real-time support is useful for mobile-phone communications, for playing audio and video, and for the signal processing in GPS receivers and radar systems, MontaVista programmer Sven-Thorsten Dietrich said in an announcement about the MontaVista real-time software sent to the Linux kernel mailing list.

"Many times, these systems have task-level response requirements in the tens to hundreds of microsecond ranges, which is a level of guaranteed task response not achievable with current 2.6 Linux technology," Dietrich said.

Hard real-time systems are often required in electronics that govern life-or-death situations. "If you change the shape of the wing surface on a fly-by-wire plane, you don't want the computer to effectively say, 'I'll get to that in a second,'" Illuminata analyst Gordon Haff said.

But there's a price to pay with hard real-time operating systems: a slower average response time than a conventional environment. And often, a regular operating system is good enough, especially if it has plenty of extra processing capacity to ensure that it doesn't get dangerously overtaxed, Haff said.

"As hardware become cheaper, overprovisioning hardware becomes an alternative in many cases to a hard real-time operating system," Haff said.

One person who still needs some convincing of the mainstream merits of the hard real-time approach is Linus Torvalds, the founder and leader of Linux.

"Almost nobody wants hard real-time, even in embedded devices," Torvalds said in an email interview. Adding the feature makes the operating system more complex and burdens the process of "locking," in which the operating system assures that different processes don't step on each others' toes when vying for the same resources.

Asked whether MontaVista's proposed software could be accepted into the main kernel, he said, "I personally think it's too intrusive, at least at this point," though it might be possible to merge the patch into the kernel in smaller pieces.

But the company has had some success in getting its ideas accepted. MontaVista programmers wrote a "pre-emption" patch useful in embedded devices that's now part of the main kernel -- not a mandatory component, but an option that can be selected.

MontaVista isn't alone in the real-time market. For starters, there are traditional embedded operating-system makers such as Wind River Systems and Green Hills Software.

Wind River, the leader in embedded computing, shunned Linux for years but now has a partnership with Red Hat to enter that part of the embedded market. And FSMLabs sells a hybrid system that lets Linux run on top of a separate hard real-time foundation.