OpenZFS 8199 - multi-threaded dmu_object_alloc()

dmu_object_alloc() is single-threaded, so when multiple threads are
creating files in a single filesystem, they spend a lot of time waiting
for the os_obj_lock.  To improve performance of multi-threaded file
creation, we must make dmu_object_alloc() typically not grab any
filesystem-wide locks.

The solution is to have a "next object to allocate" for each CPU. Each
of these "next object"s is in a different block of the dnode object, so
that concurrent allocation holds dnodes in different dbufs.  When a
thread's "next object" reaches the end of a chunk of objects (by default
4 blocks worth -- 128 dnodes), it will be reset to the per-objset
os_obj_next, which will be increased by a chunk of objects (128).  Only
when manipulating the os_obj_next will we need to grab the os_obj_lock.
This decreases lock contention dramatically, because each thread only
needs to grab the os_obj_lock briefly, once per 128 allocations.

This results in a 70% performance improvement to multi-threaded object
creation (where each thread is creating objects in its own directory),
from 67,000/sec to 115,000/sec, with 8 CPUs.

Work sponsored by Intel Corp.

Authored by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Ned Bass <bass6@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>

OpenZFS-issue: https://www.illumos.org/issues/8199
OpenZFS-commit: https://github.com/openzfs/openzfs/pull/374
Closes #4703
Closes #6117

module/zfs/zfs_znode.c

@@ -1779,14 +1779,6 @@ zfs_create_fs(objset_t *os, cred_t *cr, nvlist_t *zplprops, dmu_tx_t *tx)
 	    DMU_OT_NONE, 0, tx);
 	ASSERT(error == 0);
 
-	/*
-	 * Give dmu_object_alloc() a hint about where to start
-	 * allocating new objects. Otherwise, since the metadnode's
-	 * dnode_phys_t structure isn't initialized yet, dmu_object_next()
-	 * would fail and we'd have to skip to the next dnode block.
-	 */
-	os->os_obj_next = moid + 1;
-
 	/*
 	 * Set starting attributes.
 	 */