path: root/doc/developer-guide
diff options
authorAmar Tumballi <>2019-01-04 07:04:50 +0000
committerAmar Tumballi <>2019-01-08 11:16:03 +0000
commit37653efdc7681d1b0f255054ec2f9c9ddd4c8b14 (patch)
tree61688051f8f374ea6a1b661ef357844a477361f6 /doc/developer-guide
parent054c7ea91603acfcb01db8455b25dda7e5e831b2 (diff)
Revert "iobuf: Get rid of pre allocated iobuf_pool and use per thread mem pool"
This reverts commit b87c397091bac6a4a6dec4e45a7671fad4a11770. There seems to be some performance regression with the patch and hence recommended to have it reverted. Updates: #325 Change-Id: Id85d6203173a44fad6cf51d39b3e96f37afcec09
Diffstat (limited to 'doc/developer-guide')
1 files changed, 138 insertions, 14 deletions
diff --git a/doc/developer-guide/ b/doc/developer-guide/
index fdbbad7..5f521f1 100644
--- a/doc/developer-guide/
+++ b/doc/developer-guide/
@@ -2,13 +2,31 @@
Short for IO Buffer. It is one allocatable unit for the consumers of the IOBUF
-API, each unit hosts @page_size 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.
+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 */
-Iobufs is allocated from the per thread mem pool.
+ void *free_ptr; /* in case of stdalloc, this is the
+ one to be freed not the *ptr */
There may be need of multiple iobufs for a single fop, like in vectored read/write.
@@ -22,9 +40,104 @@ struct iobref {
int used; /* number of iobufs added to this iobref */
+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 */
-This is just a wrapper structure to keep count of active iobufs, iobuf mem pool
-alloc misses and hits.
+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).
@@ -44,15 +157,23 @@ 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 size in the mem pool(1MB as of yet))
+if (requested iobuf size > Max iobuf size in the pool(1MB as of yet))
- Perform standard allocation(CALLOC) of the requested size
+ Perform standard allocation(CALLOC) of the requested size and
+ add it to the list iobuf_pool->arenas[IOBUF_ARENA_MAX_INDEX].
- -request for memory from the per thread mem pool. This can be a miss
- or hit, based on the availablility in the mem pool. Record the hit/miss
- in the iobuf_pool.
+ -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
@@ -76,6 +197,8 @@ 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.
@@ -126,7 +249,8 @@ Unreference all the iobufs in the iobref, and also unref the iobref.
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.
+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/ 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