doc: iobuf and iobref documentation.

Change-Id: Ib6d8121a968e8160c587a8553b7333262bb44975
BUG: 1118309
Signed-off-by: Poornima G <pgurusid@redhat.com>
Reviewed-on: http://review.gluster.org/8818
Reviewed-by: Prashanth Pai <ppai@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>

1 files changed, 259 insertions, 0 deletions

diff --git a/doc/data-structures/iobuf.md b/doc/data-structures/iobuf.md
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+#Iobuf-pool
+##Datastructures
+###iobuf
+Short for IO Buffer. It is one allocatable unit for the consumers of the IOBUF
+API, each unit hosts @page_size(defined in arena structure) bytes of memory. As
+initial step of processing a fop, the IO buffer passed onto GlusterFS by the
+other applications (FUSE VFS/ Applications using gfapi) is copied into GlusterFS
+space i.e. iobufs. Hence Iobufs are mostly allocated/deallocated in Fuse, gfapi,
+protocol xlators, and also in performance xlators to cache the IO buffers etc.
+```
+struct iobuf {
+        union {
+                struct list_head      list;
+                struct {
+                        struct iobuf *next;
+                        struct iobuf *prev;
+                };
+        };
+        struct iobuf_arena  *iobuf_arena;
+
+        gf_lock_t            lock; /* for ->ptr and ->ref */
+        int                  ref;  /* 0 == passive, >0 == active */
+
+        void                *ptr;  /* usable memory region by the consumer */
+
+        void                *free_ptr; /* in case of stdalloc, this is the
+                                          one to be freed not the *ptr */
+};
+```
+
+###iobref
+There may be need of multiple iobufs for a single fop, like in vectored read/write.
+Hence multiple iobufs(default 16) are encapsulated under one iobref.
+```
+struct iobref {
+        gf_lock_t          lock;
+        int                ref;
+        struct iobuf     **iobrefs; /* list of iobufs */
+        int                alloced; /* 16 by default, grows as required */
+        int                used;    /* number of iobufs added to this iobref */
+};
+```
+###iobuf_arenas
+One region of memory MMAPed from the operating system. Each region MMAPs
+@arena_size bytes of memory, and hosts @arena_size / @page_size IOBUFs.
+The same sized iobufs are grouped into one arena, for sanity of access.
+
+```
+struct iobuf_arena {
+        union {
+                struct list_head            list;
+                struct {
+                        struct iobuf_arena *next;
+                        struct iobuf_arena *prev;
+                };
+        };
+
+        size_t              page_size;  /* size of all iobufs in this arena */
+        size_t              arena_size; /* this is equal to
+                                           (iobuf_pool->arena_size / page_size)
+                                           * page_size */
+        size_t              page_count;
+
+        struct iobuf_pool  *iobuf_pool;
+
+        void               *mem_base;
+        struct iobuf       *iobufs;     /* allocated iobufs list */
+
+        int                 active_cnt;
+        struct iobuf        active;     /* head node iobuf
+                                           (unused by itself) */
+        int                 passive_cnt;
+        struct iobuf        passive;    /* head node iobuf
+                                           (unused by itself) */
+        uint64_t            alloc_cnt;  /* total allocs in this pool */
+        int                 max_active; /* max active buffers at a given time */
+};
+
+```
+###iobuf_pool
+Pool of Iobufs. As there may be many Io buffers required by the filesystem,
+a pool of iobufs are preallocated and kept, if these preallocated ones are
+exhausted only then the standard malloc/free is called, thus improving the
+performance. Iobuf pool is generally one per process, allocated during
+glusterfs_ctx_t init (glusterfs_ctx_defaults_init), currently the preallocated
+iobuf pool memory is freed on process exit. Iobuf pool is globally accessible
+across GlusterFs, hence iobufs allocated by any xlator can be accessed by any
+other xlators(unless iobuf is not passed).
+```
+struct iobuf_pool {
+        pthread_mutex_t     mutex;
+        size_t              arena_size; /* size of memory region in
+                                           arena */
+        size_t              default_page_size; /* default size of iobuf */
+
+        int                 arena_cnt;
+        struct list_head    arenas[GF_VARIABLE_IOBUF_COUNT];
+        /* array of arenas. Each element of the array is a list of arenas
+           holding iobufs of particular page_size */
+
+        struct list_head    filled[GF_VARIABLE_IOBUF_COUNT];
+        /* array of arenas without free iobufs */
+
+        struct list_head    purge[GF_VARIABLE_IOBUF_COUNT];
+        /* array of of arenas which can be purged */
+
+        uint64_t            request_misses; /* mostly the requests for higher
+                                               value of iobufs */
+};
+```
+~~~
+The default size of the iobuf_pool(as of yet):
+1024 iobufs of 128Bytes = 128KB
+512  iobufs of 512Bytes = 256KB
+512  iobufs of 2KB      = 1MB
+128  iobufs of 8KB      = 1MB
+64   iobufs of 32KB     = 2MB
+32   iobufs of 128KB    = 4MB
+8    iobufs of 256KB    = 2MB
+2    iobufs of 1MB      = 2MB
+Total ~13MB
+~~~
+As seen in the datastructure iobuf_pool has 3 arena lists.
+
+- arenas:  
+The arenas allocated during iobuf_pool create, are part of this list. This list
+also contains arenas that are partially filled i.e. contain few active and few
+passive iobufs (passive_cnt !=0, active_cnt!=0 except for initially allocated
+arenas). There will be by default 8 arenas of the sizes mentioned above.
+- filled:  
+If all the iobufs in the arena are filled(passive_cnt = 0), the arena is moved
+to the filled list. If any of the iobufs from the filled arena is iobuf_put,
+then the arena moves back to the 'arenas' list.
+- purge:  
+If there are no active iobufs in the arena(active_cnt = 0), the arena is moved
+to purge list. iobuf_put() triggers destruction of the arenas in this list. The
+arenas in the purge list are destroyed only if there is  atleast one arena in
+'arenas' list, that way there won't be spurious mmap/unmap of buffers.
+(e.g: If there is an arena (page_size=128KB, count=32) in purge list, this arena
+is destroyed(munmap) only if there is an arena in 'arenas' list with page_size=128KB).
+
+##APIs
+###iobuf_get
+
+```
+struct iobuf *iobuf_get (struct iobuf_pool *iobuf_pool);
+```
+Creates a new iobuf of the default page size(128KB hard coded as of yet).
+Also takes a reference(increments ref count), hence no need of doing it
+explicitly after getting iobuf.
+
+###iobuf_get2
+
+```
+struct iobuf * iobuf_get2 (struct iobuf_pool *iobuf_pool, size_t page_size);
+```
+Creates a new iobuf of a specified page size, if page_size=0 default page size
+is considered.
+```
+if (requested iobuf size > Max iobuf size in the pool(1MB as of yet))
+  {
+     Perform standard allocation(CALLOC) of the requested size and
+     add it to the list iobuf_pool->arenas[IOBUF_ARENA_MAX_INDEX].
+  }
+  else
+  {
+     -Round the page size to match the stndard sizes in iobuf pool.
+     (eg: if 3KB is requested, it is rounded to 8KB).
+     -Select the arena list corresponding to the rounded size
+     (eg: select 8KB arena)
+     If the selected arena has passive count > 0, then return the
+     iobuf from this arena, set the counters(passive/active/etc.)
+     appropriately.
+     else the arena is full, allocate new arena with rounded size
+     and standard page numbers and add to the arena list
+     (eg: 128 iobufs of 8KB is allocated).
+  }
+```
+Also takes a reference(increments ref count), hence no need of doing it
+explicitly after getting iobuf.
+
+###iobuf_ref
+
+```
+struct iobuf *iobuf_ref (struct iobuf *iobuf);
+```
+  Take a reference on the iobuf. If using an iobuf allocated by some other
+xlator/function/, its a good practice to take a reference so that iobuf is not
+deleted by the allocator.
+
+###iobuf_unref
+```
+void iobuf_unref (struct iobuf *iobuf);
+```
+Unreference the iobuf, if the ref count is zero iobuf is considered free.
+
+```
+  -Delete the iobuf, if allocated from standard alloc and return.  
+  -set the active/passive count appropriately.  
+  -if passive count > 0 then add the arena to 'arena' list.  
+  -if active count = 0 then add the arena to 'purge' list.  
+```
+Every iobuf_ref should have a corresponding iobuf_unref, and also every
+iobuf_get/2 should have a correspondning iobuf_unref.
+
+###iobref_new
+```
+struct iobref *iobref_new ();
+```
+Creates a new iobref structure and returns its pointer.
+
+###iobref_ref
+```
+struct iobref *iobref_ref (struct iobref *iobref);
+```
+Take a reference on the iobref.
+
+###iobref_unref
+```
+void iobref_unref (struct iobref *iobref);
+```
+Decrements the reference count of the iobref. If the ref count is 0, then unref
+all the iobufs(iobuf_unref) in the iobref, and destroy the iobref.
+
+###iobref_add
+```
+int iobref_add (struct iobref *iobref, struct iobuf *iobuf);
+```
+Adds the given iobuf into the iobref, it takes a ref on the iobuf before adding
+it, hence explicit iobuf_ref is not required if adding to the iobref.
+
+###iobref_merge
+```
+int iobref_merge (struct iobref *to, struct iobref *from);
+```
+Adds all the iobufs in the 'from' iobref to the 'to' iobref. Merge will not
+cause the delete of the 'from' iobref, therefore it will result in another ref
+on all the iobufs added to the 'to' iobref. Hence iobref_unref should be
+performed both on 'from' and 'to' iobrefs (performing iobref_unref only on 'to'
+will not free the iobufs and may result in leak).
+
+###iobref_clear
+```
+void iobref_clear (struct iobref *iobref);
+```
+Unreference all the iobufs in the iobref, and also unref the iobref.
+
+##Iobuf Leaks
+If all iobuf_refs/iobuf_new do not have correspondning iobuf_unref, then the
+iobufs are not freed and recurring execution of such code path may lead to huge
+memory leaks. The easiest way to identify if a memory leak is caused by iobufs
+is to take a statedump. If the statedump shows a lot of filled arenas then it is
+a sure sign of leak. Refer doc/debugging/statedump.md for more details.
+
+If iobufs are leaking, the next step is to find where the iobuf_unref went
+missing. There is no standard/easy way of debugging this, code reading and logs
+are the only ways. If there is a liberty to reproduce the memory leak at will,
+then logs(gf_callinginfo) in iobuf_ref/unref might help.  
+TODO: A easier way to debug iobuf leaks.