main/stasis: Allow subscriptions to use a threadpool for message delivery

Prior to this patch, all Stasis subscriptions would receive a dedicated
thread for servicing published messages. In contrast, prior to r400178
(see review https://reviewboard.asterisk.org/r/2881/), the subscriptions
shared a thread pool. It was discovered during some initial work on Stasis
that, for a low subscription count with high message throughput, the
threadpool was not as performant as simply having a dedicated thread per
subscriber.

For situations where a subscriber receives a substantial number of messages
and is always present, the model of having a dedicated thread per subscriber
makes sense. While we still have plenty of subscriptions that would follow
this model, e.g., AMI, CDRs, CEL, etc., there are plenty that also fall into
the following two categories:
* Large number of subscriptions, specifically those tied to endpoints/peers.
* Low number of messages. Some subscriptions exist specifically to coordinate
  a single message - the subscription is created, a message is published, the
  delivery is synchronized, and the subscription is destroyed.
In both of the latter two cases, creating a dedicated thread is wasteful (and
in the case of a large number of peers/endpoints, harmful). In those cases,
having shared delivery threads is far more performant.

This patch adds the ability of a subscriber to Stasis to choose whether or not
their messages are dispatched on a dedicated thread or on a threadpool. The
threadpool is configurable through stasis.conf.

Review: https://reviewboard.asterisk.org/r/4193

ASTERISK-24533 #close
Reported by: xrobau
Tested by: xrobau
........

Merged revisions 428681 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@428687 65c4cc65-6c06-0410-ace0-fbb531ad65f3

1 files changed, 7 insertions, 0 deletions

diff --git a/UPGRADE.txt b/UPGRADE.txt
index f00983176..9720bf0d0 100644
--- a/UPGRADE.txt
+++ b/UPGRADE.txt
@@ -23,6 +23,13 @@
 
 From 13.0.0 to 13.1.0:
 
+Core:
+ - The core of Asterisk uses a message bus called "Stasis" to distribute
+   information to internal components. For performance reasons, the message
+   distribution was modified to make use of a thread pool instead of a
+   dedicated thread per consumer in certain cases. The initial settings for
+   the thread pool can now be configured in 'stasis.conf'.
+
 PJSIP:
  - Added the pjsip.conf system type disable_tcp_switch option.  The option
    allows the user to disable switching from UDP to TCP transports described