[asterisk-dev] Performance Note: Creating Local channels with ARI

[George Joseph]

Over the past week we've noticed that, when bridging a local channel
created with ARI, audio processing uses excessive amounts of CPU time.
After some digging, we determined the cause, have some recommendations, and
have a patch up for review.

The cause:

When you create a local channel via ARI and don't specify an "originator"
channel, there's no information available to limit the codecs (actually
formats) that the local channel can use.  As a result, we add ALL audio
formats to the channel's capability list.  When it comes time to make the
local channel compatible with another channel, like adding it to a bridge,
the make-compatible code chooses the best format available to preserve
quality.  Since all audio formats were in the local channel's capability
list this results in SLIN @ 192KHz being used.  While this is best for
preserving quality, it's the worst for performance and overkill for most
applications.

Recommendations:

If you need a local channel, create it after a remote channel like SIP or
PJSIP and specify the remote channel as the local channel's originator.
When this happens, the local channel's capability list will be limited to
the remote channel's capability list.  If you have 2 local channels, you'll
probably need the patch...

Patch:

There's a patch [1] up for review that adds a new parameter 'formats' to
the ARI channels object.  With it you can specify a comma-separated list of
the formats that this channel can use if an originator isn't available.
For instance, if you know that your environment needs only ulaw, you can
specify 'formats: "ulaw"' in the JSON for '/ari/channels'.  This prevents
the addition of all audio formats to the capability list.

Codec vs Format:

Sometimes we use the terms interchangeably but a format is basically codec
plus sample rate.   For instance, the signed linear 'slin' codec could use
many sample rates from the default of 8KHz up to 192KHz.  The format is
specific though: slin16 is signed linear at 16KHz.  With this patch, the
CLI command 'core show codecs' will show the default formats associated
with each codec.

[1] https://gerrit.asterisk.org/2917

-- 
George Joseph
Digium, Inc. | Software Developer
445 Jan Davis Drive NW - Huntsville, AL 35806 - US
Check us out at: www.digium.com & www.asterisk.org
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