Fix High CPU Usage & Performance Tuning

Asterisk is inherently CPU-intensive because it processes real-time audio streams. Unlike a web server that handles bursty traffic, Asterisk must process audio packets every 20ms per call without interruption. The main CPU consumers are:

Transcoding occurs when two endpoints use different codecs, forcing Asterisk to convert audio between formats in real-time. For example, if one side uses G.729 and the other uses G.711 (ulaw), Asterisk must decode G.729 and re-encode to G.711 for every single audio packet (50 packets/second per call).

Detect Transcoding:

# Show active channels with codec info
asterisk -rx "core show channels verbose"

# Look for lines like:
# SIP/100-00000001  (ulaw)    SIP/trunk-00000002  (g729)
#                    ^^^^                           ^^^^
# Different codecs = TRANSCODING happening!

# Check codec translation paths
asterisk -rx "core show translation"

# This shows a matrix of codec-to-codec conversion costs
# Lower numbers = less CPU; "n/a" = not possible

Fix: Match Codecs Everywhere

; In sip.conf - set the SAME codecs for all peers and trunks:
[general]
disallow=all
allow=alaw
allow=ulaw
; Only add g729 if ALL endpoints support it
; Avoid mixing g729 with ulaw/alaw

; For PJSIP endpoints:
[my-endpoint]
type=endpoint
disallow=all
allow=ulaw
allow=alaw
; Use the same codec list for ALL endpoints to avoid transcoding

Hardware Codec Boards:

For deployments that must transcode (e.g., G.729 trunks to G.711 phones), consider hardware transcoding cards from Sangoma/Digium (TC400B) that offload codec conversion from the CPU. These can handle hundreds of simultaneous transcoding sessions.

Complex or poorly written dialplans can cause excessive CPU usage, especially with loops, heavy database lookups, or AGI scripts that run on every call.

Common Dialplan Problems:

• Infinite loops: GoTo/GotoIf that loops back without a proper exit condition
• Excessive AGI calls: Spawning a new process for every call (use FastAGI instead)
• Heavy ODBC/database lookups: Querying a slow database on every dialplan step
• Regex pattern matching: Overly complex patterns like _X.! that match everything
• Unnecessary logging: Verbose() and NoOp() with high verbosity in production

Optimize Dialplan:

; BAD: AGI script for every call (spawns new process each time)
exten => _X.,1,AGI(my-script.php)

; GOOD: Use FastAGI (persistent connection, no process spawn)
exten => _X.,1,AGI(agi://127.0.0.1:4573/my-script)

; BAD: Catch-all pattern with heavy processing
exten => _X.!,1,Set(RESULT=${ODBC_LOOKUP(${EXTEN})})

; GOOD: Specific patterns, cache results
exten => _1NXXNXXXXXX,1,Set(RESULT=${ODBC_LOOKUP(${EXTEN})})
exten => _NXXNXXXXXX,1,Goto(${CONTEXT},1${EXTEN},1)

; BAD: Verbose logging in production
exten => _X.,n,Verbose(5,Processing call to ${EXTEN} from ${CALLERID(all)})

; GOOD: Remove or reduce verbose level in production
; exten => _X.,n,NoOp()  ; Comment out in production

# Current system overview
asterisk -rx "core show sysinfo"
asterisk -rx "core show uptime"

# Active calls and channels
asterisk -rx "core show calls"
asterisk -rx "core show channels count"
asterisk -rx "core show channels verbose"

# Check which codecs are in use (detect transcoding)
asterisk -rx "core show channels verbose" | grep -i "transcode"

# Check translation costs
asterisk -rx "core show translation recalc 10"

# Monitor from Linux side
top -p $(pidof asterisk)
htop -p $(pidof asterisk)

# Profile Asterisk CPU usage
asterisk -rx "core show profile"

# Check thread count (high thread count = potential issue)
ps -eLf | grep asterisk | wc -l

# Memory usage
asterisk -rx "core show sysinfo" | grep -i mem

# Check for process limits
cat /proc/$(pidof asterisk)/limits
ulimit -a

asterisk.conf Optimizations:

[options]
; Increase max calls (default 0 = unlimited, but set a sane limit)
maxcalls=500

; Increase max file descriptors
maxfiles=16384

; Reduce debug/verbose in production
verbose=0
debug=0

; Enable core dumps for crash analysis
dumpcore=yes

; Set process priority (higher = more CPU time, range -20 to 19)
; highpriority=yes  ; Runs with nice -10

Linux System Tuning:

# Increase file descriptor limits for Asterisk
echo "asterisk soft nofile 65536" >> /etc/security/limits.conf
echo "asterisk hard nofile 65536" >> /etc/security/limits.conf

# Increase UDP buffer sizes for RTP
echo "net.core.rmem_max=16777216" >> /etc/sysctl.conf
echo "net.core.wmem_max=16777216" >> /etc/sysctl.conf
echo "net.core.rmem_default=1048576" >> /etc/sysctl.conf
sysctl -p

# Set Asterisk process to real-time priority
chrt -f 1 $(pidof asterisk)

# Pin Asterisk to specific CPU cores (NUMA aware)
taskset -cp 0-3 $(pidof asterisk)

Reduce AMI/ARI Event Flooding:

; In manager.conf - limit event types per AMI user:
[my-ami-user]
secret=mypassword
read=system,call
write=system,call
; Do NOT use: read=all,write=all (generates massive event traffic)
; Specifically exclude: cdr,dialplan,dtmf,reporting if not needed

; Limit AMI connections
[general]
; Only allow connections from specific IPs
bindaddr=127.0.0.1
; Or use ACLs:
; permit=127.0.0.1/255.255.255.255
; deny=0.0.0.0/0.0.0.0