pluggable schedulers vs. tunable schedulers

Posted Sep 12, 2009 15:28 UTC (Sat) by mingo (subscriber, #31122)
In reply to: pluggable schedulers vs. tunable schedulers by paragw
Parent article: BFS vs. mainline scheduler benchmarks and measurements

What i believe you are missing relates to the very first question i asked: wouldnt it be better if a scheduler had nice runtime tunables that achieved the same?

Your original answer was (in part and way up in the discussion):

If we had a nice modular scheduler interface that allows us to load a scheduler at runtime or choose which scheduler to use at boot time or runtime that would solve the complexity problem and it will work well for the workloads it was designed for. As a bonus I will not have to make decisions on values of tunables - we can make the particular scheduler implementation make reasonable assumptions for the workload it was servicing.

What you are missing is that 'boot time' or 'build time' schedulers (i.e. what PlugSched did in essence) are build time / boot time tunables. A complex one but still a knob as far as the user is concerned.

Furthermore they are worse tunables than nice runtime tunables. They inconvenience the user and they inconvenince the distro. Flipping to another scheduler would force a reboot. Why do that?

For example, it does not allow the example i suggested: to run Firefox under BFS while Thunderbird under another scheduler.

So build-time/boot-time pluggable schedulers have various clear usage disadvantages and there are also have various things they cannot do.

So if you want tunability then i cannot understand why you are arguing for the technically worse solution - for a build time or boot time solution - versus a nice runtime solution.

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pluggable schedulers vs. tunable schedulers

Posted Sep 12, 2009 18:48 UTC (Sat) by paragw (guest, #45306) [Link]

Surely one single tunable (I want the desktop scheduler for example in the case of PlugSched) is better (i.e. less complex) from user standpoint rather than having to figure out say 5 complex numerical things such as granularity and what not?

Or do we have one single tunable for CFS that converts it into desktop friendly? If it does have such a knob then the next and most important question is how well does it work for desktops. From the reports I think we are still some way from claiming excellent "automatic" interactivity for desktops. Note that I am excluding the nicing games and making the user do a complex dance of figuring out how to make his/her desktop interactive. I am sure you agree that does not work well.

To your point, if we have to have one tunable for the CFS scheduler to make it desktop friendly - essentially a single knob (like sched=desktop in the PlugSched case) it is easy to see how that would fail to work satisfactorily for all desktop workloads. For one thing unless the user messes with nice levels of each process that he/she opens, minimizes or closes or brings to foreground (that is out of question from usability standpoint) the scheduler has no way to distinguish the foreground process from a background one, it has no way of distinguishing mplayer from dekstop window manager from some system daemon going bad and eating CPU.

For another, the scheduler seems to have no reliable way to know what processes it needs to favor. Window manager and the process of the foreground window need to be registered with the scheduler as foreground processes, each minimized window needs to be registered with scheduler as background. Then as long as the window manager and the process owning the foreground window are not runnable everyone else gets CPU. Multimedia applications need to be registered with the scheduler as such - automatically, so that Mplayer always gets CPU when it needs it, even favoring it over the window manager and other process of another foreground window if there is only one available CPU. Until this co-ordination happens I think we will be away from achieving great desktop interactivity which works for most desktop workloads.

Then the question would be that do we want to put all this "only needed on desktop" complexity into the completely fair scheduler or do we want to keep both separate. That is sort of a secondary question - the first question is how do we get the desktop to hint the scheduler as to which processes the user is actively interacting with, which ones are the ones he/she is likely to interact with (minimized windows) and then the scheduler favoring those accordingly - that ought to solve the interactivity problems in an automatic fashion.

[ Windows has this notion of distinguishing between "Programs" (which are running desktop applications) and background services (things without desktop interaction) and in its default configuration on the desktop it favors "Programs" and on Servers it favors "Background services" (Web Server service for e.g.). And it certainly helps interactivity. It can do this because it can distinguish between what is a desktop application and which is foreground or background and what is a non-desktop, background application.]