Re: [report] renicing X, cfs-v5 vs sd-0.46

Gene Heskett
2007-04-23T17:20:10Z

On Monday 23 April 2007, Linus Torvalds wrote:
>On Mon, 23 Apr 2007, Ingo Molnar wrote:
>> You are completely right in the case of traditional schedulers.
>
>And apparently I'm completely right with CFS too.
>
>> Using CFS-v5, with Xorg at nice 0, the context-switch rate is low:
>>
>> procs -----------memory---------- ---swap-- -----io---- --system--
>> -----cpu------ r b swpd free buff cache si so bi bo in
>> cs us sy id wa st 2 0 0 472132 13712 178604 0 0 0 32
>> 113 170 83 17 0 0 0 2 0 0 472172 13712 178604 0 0 0
>> 0 112 184 85 15 0 0 0 2 0 0 472196 13712 178604 0 0
>> 0 0 108 162 83 17 0 0 0 1 0 0 472076 13712 178604 0
>> 0 0 0 115 189 86 14 0 0 0
>
>Around 170 context switches per second.
>
>> Renicing X to -10 increases context-switching, but not dramatically so,
>> because it is throttled by CFS:
>>
>> procs -----------memory---------- ---swap-- -----io---- --system--
>> -----cpu------ r b swpd free buff cache si so bi bo in
>> cs us sy id wa st 4 0 0 475752 13492 176320 0 0 0 64
>> 116 1498 85 15 0 0 0 4 0 0 475752 13492 176320 0 0 0
>> 0 107 1488 84 16 0 0 0 4 0 0 475752 13492 176320 0 0
>> 0 0 140 1514 86 14 0 0 0 4 0 0 475752 13492 176320 0
>> 0 0 0 107 1477 85 15 0 0 0 4 0 0 475752 13492 176320
>> 0 0 0 0 122 1498 84 16 0 0 0
>
>Did you even *look* at your own numbers? Maybe you looked at "interrpts".
>The context switch numbers go from 170 per second, to 1500 per second!
>
>If that's not "dramatically so", I don't know what is! Just how many
>orders of magnitude worse does it have to be, to be "dramatic"? Apparently
>one order of magnitude isn't "dramatic"?
>
>So you were wrong. The fact that it was still "usable" is a good
>indication, but how about just admitting that you were wrong, and that
>renicing X is the *WRONG*THING*TO*DO*.
>
>Just don't do it. It's wrong. It was wrong with the old schedulers, it's
>wrong with the new scheduler, it's just WRONG.
>
>It was a hack, and it's a failed hack. And the fact that you don't seem to
>realize that it's a failure, even when your OWN numbers clearly show that
>it's failed, is a bit scary.
>
> Linus

This message prompted me to do some checking in re context switches myself,
and I've come to the conclusion that there could be a bug in vmstat itself.

Run singly the context switching is reasonable even for a -19 niceness of x,
its only showing about 200 or so on the first loop of vmstat. But throw in
the -n 1 arguments and it goes crazy on the second and subsequent loops.

X nice=0
[root@coyote ~]# vmstat -n 1
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
r b swpd free buff cache si so bi bo in cs us sy id wa
st
3 0 324 62836 37952 518080 0 0 786 446 474 201 10 4 82 4
0
0 0 324 62712 37952 518080 0 0 0 0 1309 2361 2 5 93 0
0
2 0 324 62712 37952 518080 0 0 0 0 1275 2203 2 4 94 0
0
0 0 324 62744 37952 518080 0 0 0 0 1305 2224 1 2 97 0
0
0 0 324 62744 37952 518080 0 0 0 0 1291 2232 0 1 99 0
0

X nice=-10
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
r b swpd free buff cache si so bi bo in cs us sy id wa
st
3 0 324 62432 38052 518080 0 0 784 445 476 205 10 4 82 4
0
0 0 324 62432 38052 518080 0 0 0 0 1190 3223 1 1 98 0
0
2 0 324 62440 38052 518080 0 0 0 0 1209 3210 2 3 95 0
0
0 0 324 62316 38060 518080 0 0 0 232 1201 3355 3 4 92 1
0
2 0 324 62316 38060 518080 0 0 0 0 1207 2794 1 2 97 0
0

X nice=10
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
r b swpd free buff cache si so bi bo in cs us sy id wa
st
4 0 324 62372 38184 518132 0 0 783 445 477 209 10 4 82 4
0
0 0 324 62372 38192 518132 0 0 0 272 1318 2262 0 3 97 0
0
0 0 324 62372 38192 518132 0 0 0 0 1293 2249 1 4 95 0
0
0 0 324 62248 38192 518132 0 0 0 0 1280 2443 4 2 94 0
0
0 0 324 62248 38192 518132 0 0 0 4 1294 2272 0 3 97 0
0

Now, I have NDI which set of figures is the true set, but please note that in
all 3 cases the reported values for cs didn't scale up and down all that much
if separated out into 1st pass, and subsequent passes.

And, even with X nice=10, the system is still fairly smooth and usable.

This is with 2.6.21-rc7-CFS-v5 I built late last evening. At Xnice=10 I just
played a game of patience to watch the card animations and they were
absolutely acceptably smooth. (and I won it in about 112 moves :)

>From this users viewpoint, it (cfs-v5) works, and works very well indeed, and
it deserves a place as one of 3 selectable options in mainline. The other 2
being the existing mainline way, & Con K's sd-0.45 or later. Both of these
seem to be very large enhancements to the user experience over current
mainline, which I'd discuss in terms borrowed from Joanne Dow. Comparatively
speaking, mainline has a very high vacuum.

--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Jayne: "Let's move this conversation in a not-Jayne's-fault direction."
--Episode #14, "Objects in Space"