I've tested -mm2 against -mm2+noyield and -mm2+rsdl+noyield. The
noyield patch simply makes the sched_yield syscall return immediately.
Xorg and all tests are run at nice 0.
Loads:
memload: constant memcpy of 16MB buffer
execload: constant re-exec of a trivial shell script
forkload: constant fork and exit of a trivial shell script
make -j 5: hot-cache kernel build without ccache
make -j 5 ccache: hot-cache kernel build with ccache
Tests:
beryl - 3D window manager, wiggle windows, spin desktop, etc.
galeon - web browser, rapidly scrolling long web pages by grabbing
the scroll bar
mp3 - XMMS on a FUSE sshfs over wireless (during all tests)
terminal - responsiveness of ssh and local terminal sessions
mouse - responsiveness of mouse pointer
Results:
great = completely smooth
good = fully responsive
ok = visible latency
bad = becomes difficult to use (or mp3 skips)
awful = make it stop, please
-mm2 -mm2+noyield rsdl+noyield
no load
beryl great great great
galeon good good good
mp3 good good good
terminal good good good
mouse good good good
memload x10
beryl awful/bad great good
galeon good good ok/good
mp3 good good good
terminal good good good
mouse good good good
execload x10
beryl awful/bad bad/good good
galeon good bad/good ok/good
mp3 good bad good
terminal good bad/good good
mouse good bad/good good
forkload x10
beryl good good great
galeon good good ok/good
mp3 good good good
terminal good good ok/good
mouse good good good
make -j 5
beryl ok good good/great
galeon good good ok/good
mp3 good good good
terminal good good good
mouse good good good
make -j 5 ccache
beryl ok good awful
galeon good good bad
mp3 good good bad
terminal good good bad/ok
mouse good good bad/ok
make -j 5
real 8m1.857s 8m50.659s 8m9.282s
user 7m19.127s 8m3.494s 7m30.740s
sys 0m30.910s 0m33.722s 0m29.542s
make -j 5 ccache
real 2m6.182s 2m19.032s 2m1.832s
user 1m39.466s 1m48.787s 1m37.250s
sys 0m19.741s 0m22.993s 0m20.109s
There's a substantial performance hit for not yield, so we probably
want to investigate alternate semantics for it. It seems reasonable
for apps to say "let me not hog the CPU" without completely expiring
them. Imagine you're in the front of the line (aka queue) and you
spend a moment fumbling for your wallet. The polite thing to do is to
let the next guy in front. But with the current sched_yield, you go
all the way to the back of the line.
RSDL makes most of the noyield hit back in normal make and then some
with ccache. Impressive. But ccache is still destroying interactivity
somehow. The ccache effect is fairly visible even with non-parallel
'make'.
Also note I could occassionally trigger nasty multi-second pauses with
-mm2+noyield under exectest that didn't show up elsewhere. That's
probably a bug in the mainline scheduler.
--
Mathematics is the supreme nostalgia of our time.
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