kernel - Fix several usched nits

Operating Systems / DragonFlyBSD - Matthew Dillon [apollo.backplane.com] - 27 April 2018 06:38 EDT

- Fix an issue where a usched-restricted process is pulled onto a cpu that is outside of its mask. The process will immediately push itself back onto a cpu in its mask, but this bug leads to unwanted cpu ping ponging.

- Fix an issue where cpu-bound usched-restricted processes can cause other processes to be misscheduled at a lower priority than their actual priority, resulting in unexpected stuttering.

- With these changes, nice +/- priorities should work considerably better. In the normal case, alotted cpu time will be as shown below. This is intended to be non-linear:

1 RQ0N 1:42.11 forever 2 R0N 1:22.55 forever 3 R0N 0:51.46 forever 4 RQ0N 0:24.23 forever 5 R0N 0:23.12 forever 6 RQ0N 0:19.91 forever 7 RQ0N 0:09.08 forever 8 RQ0N 0:08.38 forever 9 RQ0N 0:06.46 forever 10 RQ0N 0:06.72 forever 11 RQ0N 0:04.65 forever 12 RQ0N 0:04.60 forever 13 RQ0N 0:03.04 forever 14 RQ0N 0:02.53 forever 15 RQ0N 0:02.16 forever 16 RQ0N 0:01.55 forever 17 RQ0N 0:01.25 forever 18 RQ0N 0:00.81 forever 19 RQ0N 0:00.76 forever 20 RQ0N 0:00.32 forever

- In addition, dynamic priority operation should also work better with these changes.

- WARNING! With this change, nice values actually work as intended. A nice +20 process will get very little cpu in the face of a cpu-bound nice +0 process on a cpu. Similarly, a cpu-bound nice -20 process will allow nice +0 processes to barely function on a cpu.

And if you use an even larger spread, a cpu-bound nice -20 process will not allow a nice +1 or higher process any cpu time at all.

Having multiple cores mitigations this somewhat, but users must be careful especially when specifying negative nice values for processes.

- If you run X and have jerkiness issues due to process load, you may want to give the X server itself around a nice -5. You can fiddle, but in basic testing with everything running at nice 0, the scheduler appears to do a pretty good just generally. For example, if your browser is so overloaded that it has essentially become cpu bound, it will get cpu on an equal basis with batch jobs like bulk builds.

cbd77b01f5 kernel - Fix several usched nits
sys/kern/usched_dfly.c | 57 ++++++++++++++++++++++++++++++++++----------------
1 file changed, 39 insertions(+), 18 deletions(-)

Upstream: gitweb.dragonflybsd.org


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