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SubjectRe: Scheduler latency measurements on 2.6.11.6
On Mon, Mar 28, 2005 at 09:21:03AM -0600, Eric Bambach wrote:
> No-where in your mail, unless I missed it because i've been awake for 15
> minutes now, do you mention pre-emption. One of the major latency affecting
> differences between 2.4 and 2.6 is the introduction of pre-emption and with
> 2.6.10(ish?) you can pre-empt the big kernel lock.

Sorry, you're perfectly right. I wanted to precise this first, but I forgot
it during the mail.

The reports below were obtained with CONFIG_PREEMPT and CONFIG_PREEMPT_BLK,
but the server on which I encountered the problems first is SMP without
PREEMPT. It is more difficult to get reproducable results on SMP, that's
why I launched the tests on the notebook.

> Since you are experiencing odd latencies w.r.t. syscalls, pre-emption is a
> major point to look at. In fact, the JACK and RT Linux people are working on
> problems similiar to this right now. If your program for whatever reason has
> *hard-time* latency requirements then I suggest you look at the Realtime
> Patches for Linux 2.6.

Well, I don't need such a perfect latency, the one in Linux 2.4 was
more than enough. I can even bear several tens of ms or even occasionally
a few hundreds ms, but freezing several *seconds* regularly becomes a
real problem, particularly when the process freezes longer than the
application timeouts permit !

> I would suggest you try a few more test with various combinations of
> pre-emption. The consesus in the documentation seems to be that pre-emption
> make desktops only feel snappier and "real" workloads will suffer as a
> result.

I've just finished rebuilding the kernel without PREEMPT, I first left it
enabled by accident. I've already noticed in 2.4+preempt that it sometimes
increased the time taken by certain types of processing.

Here's a new test with 2 simultaneous 'lat3 1000000' (loop 1 second then
sleep 1 ms) :
P: Date_sec.usec select loop maxlat nbmax

2: 1112024537.278 0 1000000 51 0
2: 1112024538.279 463 1000000 24648 1
2: 1112024539.279 787 1000000 51 0
2: 1112024540.280 793 1000000 82 0
2: 1112024541.281 790 1000000 51 0
2: 1112024542.282 793 1000000 8307 0
2: 1112024543.283 840 1000000 7973 0
2: 1112024544.284 840 1000000 51 0
2: 1112024545.284 841 1000000 51 0
2: 1112024546.285 839 1000000 51 0
2: 1112024547.286 840 1000000 51 0
1: 1112024548.428 0 1000006 51 0
2: 1112024548.428 841 1140890 1000597 1
1: 1112024550.034 939 1604916 1000021 1
2: 1112024550.034 605834 1000000 51 0
2: 1112024551.035 842 1000000 51 0
1: 1112024552.184 1000794 1149662 249973 1
1: 1112024553.784 599853 1000000 51 0
2: 1112024553.784 899702 1849928 1000059 1
1: 1112024555.485 795 1699739 899864 1
2: 1112024555.925 800662 1339834 399940 1
1: 1112024557.425 39976 1899874 1000060 1
2: 1112024557.425 499918 1000000 51 0
1: 1112024559.185 760628 1000000 51 0
2: 1112024559.185 804 1759937 1000067 1
1: 1112024560.886 796 1699739 899864 1
2: 1112024561.326 800665 1339795 399950 1
1: 1112024562.826 39938 1899873 1000011 1
2: 1112024562.826 499917 1000000 51 0
1: 1112024564.587 760668 1000000 8282 0
2: 1112024564.587 843 1759890 1000020 1
1: 1112024566.295 844 1707737 899874 1
2: 1112024566.635 808661 1239812 299966 1
1: 1112024568.235 39986 1899922 1000059 1
2: 1112024568.235 599854 1000000 51 0
1: 1112024569.996 760668 1000000 51 0
2: 1112024569.996 794 1759893 1000012 1
1: 1112024571.704 842 1707737 899874 1
2: 1112024572.304 808659 1500056 560210 1
2: 1112024573.305 711 1000000 51 0
I've just found something interesting when I run one process with a
1-second loop and another one in parallel with a 10-seconds loop. The
first process is heavily hindered each time the second one leaves the
select() : pauses of up to 2.5 seconds are observed, and I cannot
explain them as the max time_slice should be 800 ms. It's just as if
the second process could get 3 consecutive full time-slices + one
little :

P: Date_sec.usec select loop maxlat nbmax
1: 1112024838.490 0 1000000 52 0
1: 1112024839.491 510 1000000 51 0
1: 1112024840.492 793 1000000 24782 1
1: 1112024841.520 835 1027841 745569 2
1: 1112024842.620 99976 1000000 8287 0
1: 1112024843.621 791 1000000 51 0
1: 1112024844.628 793 1005846 99996 3
1: 1112024845.728 99930 1000000 56 0
1: 1112024846.729 792 1000000 8298 0
1: 1112024847.745 840 1015844 99996 3
1: 1112024848.746 945 1000000 99996 5
1: 1112024849.747 793 1000000 99996 5
2: 1112024850.645 0 10017626 1000070 26 => process2 sleeps
1: 1112024853.245 792 3496467 2499621 5
1: 1112024854.344 99929 1000000 8297 0
1: 1112024855.345 841 1000000 8308 0
1: 1112024856.362 792 1015845 99996 3
1: 1112024857.462 99930 1000000 51 0
1: 1112024858.463 793 1000000 51 0
1: 1112024859.464 793 1000000 99995 3
1: 1112024860.465 792 1000000 99996 5
2: 1112024860.721 99969 10016477 1000070 17 => process2 sleeps
1: 1112024863.361 793 2895559 2499631 2
1: 1112024864.461 99928 1000000 51 0
Regards,
Willy

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