doc: Add brick mux thread reduction design doc in developer guide

Change-Id: I5309efb5826d4385eadea29c1cf973f504eef9e5
Updates: #475
Signed-off-by: Poornima G <pgurusid@redhat.com>

doc/developer-guide/README.md

@@ -57,6 +57,11 @@ Translators
 -   [Storage/posix Translator](./posix.md)
 -   [Compression translator](./network_compression.md)
 
+Brick multiplex
+---------------
+
+-   [Brick mux resource reduction](./brickmux-thread-reduction.md)
+
 Fuse
 ----
 

doc/developer-guide/brickmux-thread-reduction.md

@@ -0,0 +1,64 @@
+#Resource usage reduction in brick multiplexing
+
+Each brick is regresented with a graph of translators in a brick process.
+Each translator in the graph has its own set of threads and mem pools
+and other system resources allocations. Most of the times all these
+resources are not put to full use. Reducing the resource consumption
+of each brick is a problem in itself that needs to be addressed. The other
+aspect to it is, sharing of resources across brick graph, this becomes
+critical in brick multiplexing scenario. In this document we will be discussing
+only about the threads.
+
+If a brick mux process hosts 50 bricks there are atleast 600+ threads created
+in that process. Some of these are global threads that are shared by all the
+brick graphs, and others are per translator threads. The global threads like
+synctask threads, timer threads, sigwaiter, poller etc. are configurable and
+do not needs to be reduced. The per translator threads keeps growing as the
+number of bricks in the process increases. Each brick spawns atleast 10+
+threads:
+- io-threads
+- posix threads:
+     1. Janitor
+     2. Fsyncer
+     3. Helper
+     4. aio-thread
+- changelog and bitrot threads(even when the features are not enabled)
+
+##io-threads
+
+io-threads should be made global to the process, having 16+ threads for
+each brick does not make sense. But io-thread translator is loaded in
+the graph, and the position of io-thread translator decides from when
+the fops will be parallelised across threads. We cannot entirely move
+the io-threads to libglusterfs and say the multiplexing happens from
+the master translator or so. Hence, the io-thread orchestrator code
+is moved to libglusterfs, which ensures there is only one set of
+io-threads that is shared among the io-threads translator in each brick.
+This poses performance issues due to lock-contention in the io-threds
+layer. This also shall be addressed by having multiple locks instead of
+one global lock for io-threads.
+
+##Posix threads
+Most of the posix threads execute tasks in a timely manner, hence it can be
+replaced with a timer whose handler register a task to synctask framework, once
+the task is complete, the timer is registered again. With this we can eliminate
+the need of one thread for each task. The problem with using synctasks is
+the performance impact it will have due to make/swapcontext. For task that
+does not involve network wait, we need not do makecontext, instead the task
+function with arg can be stored and executed when a synctask thread is free.
+We need to implement an api in synctask to execute atomic tasks(no network wait)
+without the overhead of make/swapcontext. This will solve the performance
+impact associated with using synctask framework.
+
+And the other challenge, is to cancel all the tasks pending from a translator.
+This is important to cleanly detach brick. For this, we need to implement an
+api in synctask that can cancel all the tasks from a given translator.
+
+For future, this will be replced to use global thread-pool(once implemented).
+
+##Changelog and bitrot threads
+
+In the initial implementation, the threads are not created if the feature is
+not enabled. We need to share threads across changelog instances if we plan
+to enable these features in brick mux scenario.
+