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+20200119 bkw: The default jack version on SlackBuilds.org is now 1.9.14,
+aka JACK2. The old 0.125.0 version (formerly jack-audio-connection-kit)
+is still available as "jack1". The JACK2 build used to be called "jack2",
+and has been renamed to simply "jack". SBo maintainers take note: please
+don't list jack1 in REQUIRES for your builds. If your build really does
+work only with jack1 and fails with jack, please contact me (B. Watson,
+yalhcru@gmail.com) and let me know the details.
+
+This information might be helpful in understanding the differences
+between jack and jack1.
+
+jack and jack1 are API compatible enough that applications can be built
+against either, and in fact most (possibly all?) apps can be built
+against one and run with the other with no problems.
+
+jack1 wasn't designed to benefit from multiple CPU cores/threads. It may
+(or may not) offer slightly better performance on single-core systems.
+
+jack no longer supports jack1's "-Z" flag.
+
+When using -Xseq with jack, connect your ALSA MIDI devices to the system
+"MIDI thru" port, then connect that port to the JACK midi capture
+port. This is an extra step that isn't necessary with jack1.
+
+jack stores a persistent "registry" and database in /dev/shm, which
+is intended to speed up jack startup and allow multiple jack servers
+on the same host to cooperate. There is one small issue with this:
+if jackd can't write to /dev/shm/jack_db/, it will fail to start
+(segfault). If this happens, make sure jackd is not running, and "rm
+-rf /dev/shm/jack*". This only happens when jackd is used by different
+users, which means most of us will be unaffected by it. Upstream has
+been notified, and a fix is being worked on.
+
+Original README from the old jack2 package has some possibly outdated
+info on the differences between 1 and 2:
+
+jackdmp (aka JACK2) is a C++ version of the JACK low-latency audio
+server for multi-processor machines. It is a new implementation
+of the JACK server core features that aims in removing some
+limitations of the JACK1 design. The activation system has been
+changed for a data flow model and lock-free programming techniques
+for graph access have been used to have a more dynamic and
+robust system.
+
+- jackdmp use a new client activation model that allows simultaneous
+client execution (on a smp machine) when parallel clients exist
+in the graph (client that have the same inputs). This activation model
+allows to better use available CPU on a smp machine, but also works
+on a mono-processor machine.
+
+- jackdmp use a lock-free way to access (read/write) the client graph,
+thus allowing connections/disconnection to be done without
+interrupting the audio stream. The result is that
+connections/disconnections are glitch-free.
+
+- jackdmp can work in 2 different modes at the server level :
+  - synchronous activation : in a given cycle, the server waits for
+    all clients to be finished (similar to normal jackd)
+  - asynchronous activation : in a given cycle, the server does not
+    wait for all clients to be finished and use output buffer
+    computed the previous cycle. 
+  
+The audible result of this mode is that if a client is not activated
+during one cycle, other clients may still run and the resulting audio
+stream will still be produced (even if its partial in some way).
+This mode usually result in fewer (less audible) audio glitches in a
+loaded system.