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hodec2: fix congestion oscillation bug 

When evenly distributing congestion across cells, count the number of
occupied lchans surpassing congestion, and not the overall number of
free lchans -- which disregards congestion thresholds.

Fix the bugs shown by
 test_congestion_no_oscillation.ho_vty
 test_balance_congestion_tchf_tchh.ho_vty

This implements a simple calculation for congestion load by counting
lchans in use above congestion. An improvement of this calculation
using percent follows in I55234c6c99eb02ceee52be0d7388bea14304930f.

Related: SYS#5259
Change-Id: Icb373dc6bfc9819446db5e96f71921781fe2026d
This commit is contained in:
Neels Hofmeyr 2021-01-05 20:09:11 +01:00
parent db52370c2e
commit 0096f27b61
4 changed files with 14 additions and 36 deletions
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15
src/osmo-bsc/handover_decision_2.c
View File

@ -443,6 +443,7 @@ static void check_requirements(struct ho_candidate *c)
{
uint8_t requirement = 0;
unsigned int penalty_time;
int current_overbooked;
c->requirements = 0;
/* Requirement A */
@ -625,14 +626,17 @@ static void check_requirements(struct ho_candidate *c)
/* Requirement C */
/* the nr of free timeslots of the target cell must be >= the
* free slots of the current cell _after_ handover/assignment */
/* the nr of lchans surpassing congestion on the target cell must be <= the lchans surpassing congestion on the
* current cell _after_ handover/assignment */
current_overbooked = c->current.min_free_tch - c->current.free_tch;
if (requirement & REQUIREMENT_A_TCHF) {
if (c->target.free_tchf - 1 >= c->current.free_tch + 1)
int target_overbooked = c->target.min_free_tchf - c->target.free_tchf;
if (target_overbooked + 1 <= current_overbooked - 1)
requirement |= REQUIREMENT_C_TCHF;
}
if (requirement & REQUIREMENT_A_TCHH) {
if (c->target.free_tchh - 1 >= c->current.free_tch + 1)
int target_overbooked = c->target.min_free_tchh - c->target.free_tchh;
if (target_overbooked + 1 <= current_overbooked - 1)
requirement |= REQUIREMENT_C_TCHH;
}
@ -1743,8 +1747,7 @@ next_c1:
goto exit;
}
LOGPHOBTS(bts, LOGL_DEBUG, "Did not find a best candidate that fulfills requirement C"
" (omitting change from AHS to AFS)\n");
LOGPHOBTS(bts, LOGL_DEBUG, "Did not find a best candidate that fulfills requirement C\n");
exit:
/* free array */

6
tests/handover/test_balance_congestion_tchf_tchh.ho_vty
View File

@ -35,8 +35,7 @@ network
set-ts-use trx 0 0 states * TCH/F TCH/F - - TCH/HH - -
meas-rep lchan * * * * rxlev 20 rxqual 0 ta 0
congestion-check
# FAIL: should be the same as above, but seeing a handover from F to H
expect-ho from lchan 0 0 1 0 to lchan 0 0 6 0
expect-no-chan
# TCH/F = +1, TCH/H = +2 above congestion. Moving a TCH/H to TCH/F would just
# reverse the situation to F=+2 H=+1. Nothing happens.
@ -49,5 +48,4 @@ expect-no-chan
set-ts-use trx 0 0 states * TCH/F TCH/F - - TCH/HH TCH/HH -
meas-rep lchan * * * * rxlev 20 rxqual 0 ta 0
congestion-check
# FAIL: should be the same as above, from lchan 0 0 5 0 to lchan 0 0 3 0, but no handover happens
expect-no-chan
expect-ho from lchan 0 0 5 0 to lchan 0 0 3 0

22
tests/handover/test_congestion_no_oscillation.ho_vty
View File

@ -1,6 +1,5 @@
# Do not oscillate handover from TCH/F to TCH/H on a neighbor due to congestion,
# and then back to the original cell due to RXLEV.
# Currently this test script shows the undesired oscillation.
create-bts trx-count 1 timeslots c+s4 TCH/F TCH/F TCH/F TCH/F TCH/F TCH/F PDCH
network
@ -25,24 +24,5 @@ expect-ts-use trx 1 0 states * TCH/F TCH/F TCH/F TCH/F TCH/H- - *
# measurements continue to be the same
meas-rep lchan 1 0 5 0 rxlev 20 rxqual 0 ta 0 neighbors 40
# FAIL: RXLEV oscillation back to bts 0
expect-ho from lchan 1 0 5 0 to lchan 0 0 2 0
expect-ts-use trx 0 0 states * TCH/F TCH/F - - - - *
expect-ts-use trx 1 0 states * TCH/F TCH/F TCH/F TCH/F - - *
meas-rep lchan 0 0 2 0 rxlev 40 rxqual 0 ta 0 neighbors 20
# despite the better RXLEV, congestion prevents oscillation back to bts 0
expect-no-chan
# FAIL: congestion oscillation again to bts 1
congestion-check
expect-ho from lchan 0 0 2 0 to lchan 1 0 5 0
expect-ts-use trx 0 0 states * TCH/F - - - - - *
expect-ts-use trx 1 0 states * TCH/F TCH/F TCH/F TCH/F TCH/H- - *
# FAIL: RXLEV oscillation back to bts 0
meas-rep lchan 1 0 5 0 rxlev 20 rxqual 0 ta 0 neighbors 40
expect-ho from lchan 1 0 5 0 to lchan 0 0 2 0
meas-rep lchan 0 0 2 0 rxlev 40 rxqual 0 ta 0 neighbors 20
# FAIL: congestion oscillation again to bts 1
congestion-check
expect-ho from lchan 0 0 2 0 to lchan 1 0 5 0

7
tests/handover/test_congestion_no_oscillation2.ho_vty
View File

@ -1,8 +1,5 @@
# Almost identical to test_amr_oscillation.ho_vty, this has just two more TCH/H slots in BTS 1, and does not trigger the
# oscillation bug. The number of free TCH/H in BTS 1 should be unrelated to the congestion status of BTS 0, which
# illustrates that the even distribution of congestion is fundamentally flawed.
# This test script shows the desired behavior, though by common sense there should be no reason why we see the bug in
# test_amr_oscillation.ho_vty and not here.
# Almost identical to test_amr_oscillation.ho_vty, this has just two more TCH/H slots in BTS 1, and did not trigger the
# oscillation bug (which has since been fixed, so that both tests behave identically now).
create-bts trx-count 1 timeslots c+s4 TCH/F TCH/F TCH/F TCH/F TCH/F TCH/F PDCH
network