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Andrew Morton And The Low-Latency Kernel Patch
An Anonymous Coward writes: "KernelTrap has interviewed Linux kernel hacker Andrew Morton, author of the low-latency patch. Though his patch has received less attention than Robert Love's preemptible kernel patch (recently merged into the 2.5 kernel), it results in quite significantly lower latencies. The interview is quite interesting, delving into the low-latency patch, explaining how it works and the differences between it and the preempt patch. He also talks about his ext3 work, porting that journaling filesystem from the older stable 2.2 kernel to the current stable 2.4 kernel."
A day in the life of a kernel hacker.
A realtime os, which usually has low latency, has nothing about the duration of latency, but rather a guarantee of latency.
For example, suppose you send a packet off into the internet, a realtime os would guarantee that the packet was sent within x number of nanoseconds. A realtime os would main this guarantee, regardless of the load on the system, the size of the packet, etc.
I've been waiting for years for Linux to have finer control of process scheduling.
I hope someday that Linux will use a method similar to Irix, where you can specify a priority from 0 to 255, modify it's timeslice, and make it realtime or timeshared. This was one of the best things about Irix, and something I could really use for Linux.
"With an internally preemptible kernel the explicit task yielding is not necessary, because the context switch is performed in the interrupt return path and via open-coded yields which are hidden in the unlock code. But you cannot preempt an in-kernel process while it holds locks, so all the unlock, relock and fixup code is needed in either approach."
:)
Try getting your head round that one when needing sleep
The difference is that hard real time doesn't mean low latency it just means that there is a _guaranteed_ maximum latency.
Soft real time means that you can almost gaurantee the latency. Generally, of course, you want these latencies to be pretty small. Soft real time is for when you use check the "use real time where available" option on xmms and run it under sudo.
I hear that Linux (probably with patches) is a little better than windows and a little worse than os X for latency.
Is there a formal difference between low latency and a realtime OS?
Yes. A realtime OS _guarantees_ that certain events trigger defined responses within specified times. A realtime OS is almost by definition an embedded OS, i.e., its hardware is rigorously specific and very tightly bound. A realtime OS also typically provides a very limited set of functions, as opposed to a general purpose OS. A low-latency OS, on the other hand, provides generalized structures for 1st-level/2nd-level interrupt handlers, real/virtual memory management, and facilities for locking, preemptive-priority dispatching, etc., but offers low latency on a merely best-efforts basis depending upon what all happens to be inflight at the moment. See the difference?
Examples of realtime systems: automotive control systems including engine power/emissions management, suspension and braking management, even airbag controls; aircraft fly-by-wire systems that control aerodynamically unstable airframes.
Examples of low-latency systems: mainframes - if you're a high-priority system task, you get _very_ low latencies - but exact timings aren't guaranteed in all situations.
IMHO, SoftUpdates are better than Journalled File Systems. There's no journal file to maintain, just careful ordering of the writes. Why no discussion of it for Linux?