12 ms·

Why would a real-time OS help at all with latency? All RT means is that the latency can be reliably upper-bounded (but note that that upper bound might be very

Why would a real-time OS help at all with latency? All RT means is that the latency can be reliably upper-bounded (but note that that upper bound might be very high/slow), it doesn't mean that the latency will be reduced. Real-time OSs aren't faster.

codys 7y ago
linux-rt is a patchset that changes the behavior of linux to increase the number of places where preemption can occur (among other things). Doing this decreases certain types of latency in certain situations. As an example, it tries to have interrupts disabled less frequently and for shorter intervals, and uses mutexes instead of spinlocks. As a result, using linux-rt can provide a lower latency experience compared plain linux.
- joosters 7y ago
  Ah, that's fair enough, but it isn't 'real time', which is the thing I was assuming from the 'RT' in the name. Perhaps linux-ll would be a better name, for 'low latency'. RT just confuses what it is trying to do.
  - codys 7y ago
    It is trying to make the linux kernel more real time capable. Having periods of time where preemption isn't enabled (due to having interrupts disabled, etc) results in more variation in when tasks are scheduled, including real time tasks. The reality is that "real time" as a definition covers many "features" and design choices because many ducks need to be in a row for real time tasks to run properly. Decreasing variation in the scheduling of (real time) tasks is one of those items. As a result, it's entirely reasonable to call "linux-rt" "linux-rt".
Dylan16807 7y ago
For something that normally takes milliseconds, it's metrics like 99% latency that matter, not average latency. It doesn't have to be "faster".
snvzz 7y ago
>reliably upper-bounded. Is extremely desirable. Those multi-ms peaks of latency Linux has are the ones that cause audio cuts and perceived hiccups. Of course it doesn't matter perceptually if the average is 1µs or 5µs. It's all about the peaks, and keeping them bounded enough so that latency does never cross the perceptual threshold.
- joosters 7y ago
  But none of the apps they would be running (their browser, in this specific case) are RT. So if the application isn't asking for a hard limit on latency, they aren't going to get anything different on a RT OS.

codys 7y ago

linux-rt is a patchset that changes the behavior of linux to increase the number of places where preemption can occur (among other things). Doing this decreases certain types of latency in certain situations. As an example, it tries to have interrupts disabled less frequently and for shorter intervals, and uses mutexes instead of spinlocks. As a result, using linux-rt can provide a lower latency experience compared plain linux.

joosters 7y ago
Ah, that's fair enough, but it isn't 'real time', which is the thing I was assuming from the 'RT' in the name. Perhaps linux-ll would be a better name, for 'low latency'. RT just confuses what it is trying to do.
- codys 7y ago
  It is trying to make the linux kernel more real time capable. Having periods of time where preemption isn't enabled (due to having interrupts disabled, etc) results in more variation in when tasks are scheduled, including real time tasks. The reality is that "real time" as a definition covers many "features" and design choices because many ducks need to be in a row for real time tasks to run properly. Decreasing variation in the scheduling of (real time) tasks is one of those items. As a result, it's entirely reasonable to call "linux-rt" "linux-rt".

Dylan16807 7y ago

For something that normally takes milliseconds, it's metrics like 99% latency that matter, not average latency. It doesn't have to be "faster".

snvzz 7y ago

>reliably upper-bounded. Is extremely desirable. Those multi-ms peaks of latency Linux has are the ones that cause audio cuts and perceived hiccups. Of course it doesn't matter perceptually if the average is 1µs or 5µs. It's all about the peaks, and keeping them bounded enough so that latency does never cross the perceptual threshold.

joosters 7y ago
But none of the apps they would be running (their browser, in this specific case) are RT. So if the application isn't asking for a hard limit on latency, they aren't going to get anything different on a RT OS.