On Tue, Oct 2, 2012 at 7:05 PM, Hank Cohen <hank.co...@altior.com> wrote:
> There is an important difference between real time and real fast > > Real time means that system response must meet a fixed schedule. > Real fast just means sooner is better. > Good thought, but real-time can also include a fixed schedule and a specified list of exceptional conditions which would prevent meeting the schedule. It may also include a fixed schedule that must be met some fraction of the time (usually very near 100% of the time). Without providing exceptions, you basically force the designer to lie about how reliable their system is. > Real time systems always have hard schedules. The schedule could be in > microseconds to control a laser for making masks for semiconductor > manufacturing, milliseconds to control the ignition in your car or flight > controls on an F-22 or seconds for even slower moving processes. In real > time system missing the schedule can mean that very bad things happen: > planes fall from the sky, your laser printer fries it's imaging drum, > factories explode etc. > It can mean that. But if you specify the exceptional situations you can specifically mitigate for them. > Most transaction processing is happy with real fast > The folks doing high velocity trading are pretty close to real time but > they probably will be happy with real fast. > If real fast systems miss a schedule then someone loses money. > Yeah. And if you talk to these guys, they know the difference and ask for real-time. > The reason that RTOS type operating systems are popular for real time > applications is that they don't allow operations to spend indeterminate > amounts of time in uninterruptable states. Java will never qualify as a > real time system because it has garbage collection and garbage collection > can lock up a system for an indefinite amount of time while it goes through > marking and counting. > You are behind the times on a few counts. - Java's collectors don't "count". - Java can be real-time: http://rtjava.blogspot.co.uk/2009/07/real-time-java-vms.html - Garbage collection can be deterministic and real-time: http://www.ibm.com/developerworks/java/library/j-rtj4/index.html http://docs.oracle.com/cd/E13221_01/wlrt/docs30/intro_wlrt/tuning.html
