llc: Use CoDel to drop packages from the LLC queue
Currently packets are only dropped if they have reached their maximum
life time. This leads to LLC queues being constantly filled under
load, increasing the latency up to the maximum life time. This kind
of bufferbloat hinders TCP's congestion avoidance algorithms. To keep
the queues short, the CoDel active queue management algorithm can be
used.
This commit changes to llc_dequeue method to apply the CoDel
algorithm to selectively drop LLC frames before they passed to the
TBF layer to be encoded in BSNs. This feature is currently disabled
by default.
The CoDel state is managed per MS since the LLC queues are also kept
in the MS objects.
Note that there is still some buffering in the TBF objects, in the
worst case (CS4) 3.5kByte + LLC-MTU octets are stored there. The
resulting additional packet delay is not (yet) taken into account for
CoDel.
Also note that configuration changes are applied to new MS objects
only.
The following VTY commands are added to the 'pcu' node:
- queue codel activates CoDel, the interval is selected by
the implementation
- queue codel interval <1-1000>
activates CoDel with a fixed interval given
in centiseconds (10ms-10s)
- no queue codel deactivates CoDel
Which interval value to use is still an open issue. For high speed
links (e.g. Ethernet), CoDel suggests 100ms. For slower links, the
expected RTT is recommended. The current implementation uses a
default value of 2000ms.
Measurements:
Note that the following measurements depend on several other factors,
most notably the interaction with the SGSN's flow control. They are
just examples to give an idea how CoDel might influence some
parameters.
The measurements have been done with a single E71, first with a
running ping only (Idle), then with an additional TCP download
of a 360k file (Busy). The CoDel interval was set to 1s.
- Idle :
ping ~400ms, avg queue delay 0ms, dropped 0
- Busy, No CoDel:
ping ~6s, avg queue delay 4-6s,
dropped 0, scheduled 948, duration 54s
- Busy, CoDel:
ping 500-1500ms, avg queue delay ~600ms,
dropped 77, scheduled 1040, duration 60s
More measurements with two MS downloading in parallel (two
independant measurements per case).
- Busy, No CoDel:
dropped 0, scheduled 1883, duration 121s
dropped 19, scheduled 2003, duration 133s
- Busy, CoDel:
dropped 22, scheduled 1926, duration 116s
dropped 22, scheduled 1955, duration 108s
Sponsored-by: On-Waves ehf
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index cdd02ba..4739a50 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -25,6 +25,7 @@
#include <gprs_rlcmac.h>
#include <gprs_debug.h>
#include <gprs_bssgp_pcu.h>
+#include <gprs_codel.h>
#include <decoding.h>
#include "pcu_utils.h"
@@ -36,6 +37,7 @@
#include <errno.h>
#include <string.h>
+#include <math.h>
/* After sending these frames, we poll for ack/nack. */
#define POLL_ACK_AFTER_FRAMES 20
@@ -249,6 +251,13 @@
gprs_bssgp_update_queue_delay(tv_recv, &tv_now);
+ if (ms() && ms()->codel_state()) {
+ int bytes = llc_queue()->octets();
+ if (gprs_codel_control(ms()->codel_state(),
+ tv_recv, &tv_now, bytes))
+ goto drop_frame;
+ }
+
/* Is the age below the low water mark? */
if (!gprs_llc_queue::is_frame_expired(&tv_now2, tv_disc))
break;
@@ -274,6 +283,7 @@
}
bts->llc_timedout_frame();
+drop_frame:
frames++;
octets += msg->len;
msgb_free(msg);