Re: BFS vs. mainline scheduler benchmarks and measurements

From: Nikos Chantziaras
Date: Sun Sep 06 2009 - 23:39:11 EST

On 09/06/2009 11:59 PM, Ingo Molnar wrote:
Also, i'd like to outline that i agree with the general goals
described by you in the BFS announcement - small desktop systems
matter more than large systems. We find it critically important
that the mainline Linux scheduler performs well on those systems
too - and if you (or anyone else) can reproduce suboptimal behavior
please let the scheduler folks know so that we can fix/improve it.

BFS improved behavior of many applications on my Intel Core 2 box in a way that can't be benchmarked. Examples:

mplayer using OpenGL renderer doesn't drop frames anymore when dragging and dropping the video window around in an OpenGL composited desktop (KDE 4.3.1). (Start moving the mplayer window around; then drop it. At the moment the move starts and at the moment you drop the window back to the desktop, there's a big frame skip as if mplayer was frozen for a bit; around 200 or 300ms.)

Composite desktop effects like zoom and fade out don't stall for sub-second periods of time while there's CPU load in the background. In other words, the desktop is more fluid and less skippy even during heavy CPU load. Moving windows around with CPU load in the background doesn't result in short skips.

LMMS (a tool utilizing real-time sound synthesis) does not produce "pops", "crackles" and drops in the sound during real-time playback due to buffer under-runs. Those problems amplify when there's heavy CPU load in the background, while with BFS heavy load doesn't produce those artifacts (though LMMS makes itself run SCHED_ISO with BFS) Also, hitting a key on the keyboard needs less time for the note to become audible when using BFS. Same should hold true for other tools who traditionally benefit from the "-rt" kernel sources.

Games like Doom 3 and such don't "freeze" periodically for small amounts of time (again for sub-second amounts) when something in the background grabs CPU time (be it my mailer checking for new mail or a cron job, or whatever.)

And, the most drastic improvement here, with BFS I can do a "make -j2" in the kernel tree and the GUI stays fluid. Without BFS, things start to lag, even with in-RAM builds (like having the whole kernel tree inside a tmpfs) and gcc running with nice 19 and ionice -c 3.

Unfortunately, I can't come up with any way to somehow benchmark all of this. There's no benchmark for "fluidity" and "responsiveness". Running the Doom 3 benchmark, or any other benchmark, doesn't say anything about responsiveness, it only measures how many frames were calculated in a specific period of time. How "stable" (with no stalls) those frames were making it to the screen is not measurable.

If BFS would imply small drops in pure performance counted in instructions per seconds, that would be a totally acceptable regression for desktop/multimedia/gaming PCs. Not for server machines, of course. However, on my machine, BFS is faster in classic workloads. When I run "make -j2" with BFS and the standard scheduler, BFS always finishes a bit faster. Not by much, but still. One thing I'm noticing here is that BFS produces 100% CPU load on each core with "make -j2" while the normal scheduler stays at about 90-95% with -j2 or higher in at least one of the cores. There seems to be under-utilization of CPU time.

Also, by searching around the net but also through discussions on various mailing lists, there seems to be a trend: the problems for some reason seem to occur more often with Intel CPUs (Core 2 chips and lower; I can't say anything about Core I7) while people on AMD CPUs mostly not being affected by most or even all of the above. (And due to this flame wars often break out, with one party accusing the other of imagining things). Can the integrated memory controller on AMD chips have something to do with this? Do AMD chips generally offer better "multithrading" behavior? Unfortunately, you didn't mention on what CPU you ran your tests. If it was AMD, it might be a good idea to run tests on Pentium and Core 2 CPUs.

For reference, my system is:

CPU: Intel Core 2 Duo E6600 (2.4GHz)
Mainboard: Asus P5E (Intel X38 chipset)
RAM: 6GB (2+2+1+1) dual channel DDR2 800
GPU: RV770 (Radeon HD4870).

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