DelayIdleScheduler experiment

I may be out of date on this but I think that the Cog setup makes it possible to have a timer check every 2mS by default. The relinquishProcessor stuff was a definite hack to try to reduce cpu usage on laptoppy things and I don't think anyone ever expected it to be used for more than 1mS sleeps. It was one of those "I wonder if this will improve anything?" ideas that got left in place.

Surely the ideal would be to
a) as you point out, add this delay to the main delay list

b) find a way to provide an outside timer to trigger the delay at the top of the list rather than relying on the vm eventually checking the tick etc.

Interestingly, we have the threaded ticker for many systems as part of the Cog vm. I don't know if it works on all systems; I do know it would be 'interesting' to implement on RISC OS, but that's life. A quick look at unix/sqUnixHeartbeat.c suggests to me that it might be plausible to use variable length beats

and tie the time to any sooner-than-2mS event wanted.

We'd (probably) change Delay handling to behave as if there were always a 1 or 2mS (fake)delay even if the nearest actual Delay were far beyond that.

> On 2018-10-07, at 1:11 AM, Ben Coman <[hidden email]> wrote:
>
> Cross posting since at this level systems are still very similar.
>
> I've sometimes wondered about how the idle process
> relinquishing the CPU interacted with delay scheduling,
> since they operate at the extreme opposite ends of process priorities.
>
> Historically there was...
> ProcessorScheduler class >> idleProcess
> [    "A default background process which is invisible."
>      [true] whileTrue:
>          [self relinquishProcessorForMicroseconds: 1000]     
>
> which had the following performance this for later comparison...
>     (1 to: 10) collect: [:i | [ (Delay forMilliseconds: 1) wait ] timeToRun asMilliSeconds].
>     ==> "#(2 5 4 2 4 4 2 4 4 3)"
>
> To improve battery life yield time Pharo changed this to 50000 in...
> * https://pharo.fogbugz.com/f/cases/20425/Yield-longer-in-ProcessorScheduler-idleProcess
> * https://github.com/pharo-project/pharo/commit/0b0d12dc
> but had a negative impact on delays...
>     (1 to: 10) collect: [:i | [ (Delay forMilliseconds: 1) wait ] timeToRun asMilliSeconds].
>    ==> "#(36 51 50 51 50 51 51 30 50 50)"
> as reported... https://pharo.fogbugz.com/f/cases/22400/Delays-are-not-working-properly
>
> The problem seems to be that #relinquishProcessorForMicroseconds: suspends the main VM thread for a fixed amount during which expired delays cannot be dealt with.  I'm not sure of the exact mechanism in the VM but this looks related...
> Win: https://github.com/OpenSmalltalk/opensmalltalk-vm/blob/e2fa2d1/platforms/win32/vm/sqWin32Window.c#L1593
> Mac: https://github.com/OpenSmalltalk/opensmalltalk-vm/blob/a8a1dc1/platforms/unix/vm/sqUnixHeartbeat.c#L263
> I'm not sure about Linux
>
> One idea was to have a setting, but another path is to mix the idle-relinquish
> into the delay-scheduling to dynamically relinquish up to exactly when the activeDelay expires.
>
> After loading...
>    https://github.com/pharo-project/pharo/pull/1887
> you can do...
>     DelayIdleTicker installExperiment.
>     (1 to: 10) collect: [:i | [ (Delay forMilliseconds: 1) wait ] timeToRun asMilliSeconds].
> ==>"#(1 1 2 2 2 2 2 2 2 2)" which btw is half the latency of the original
>     DelayIdleTicker debug: true.  "CPU usage goes way up"
>     DelayIdleTicker uninstall.
>
> I'm now seeking:
> * comment on how this might interact with parts of the VM I'm not familiar with.
> * help to discover and stress test corner cases. 
> * help to determine if there actually is a benefit and characterize what that is,
> especially anyone familiar with headless images with reduced UI and I/O events.
> I was surprised to see the existing system already often showed 0% CPU usage (in Windows).
>
> cheers -ben
>


tim
--
tim Rowledge; [hidden email]; http://www.rowledge.org/tim
Bayard(n): a person armed with the self-confidence of ignorance