DAOS-16445 client: Add function to cycle OIDs non-sequentially (#14999)

We've noticed that with sequential order, object placement is poor.

We get 40% fill for 8GiB files with 25 ranks and 16 targets per rank
with EC_2P1G8. With this patch, we get a much better distribution.

This patch adds the following:

1. A function for cycling oid.hi incrementing by a large prime
2. For DFS, randomize the starting value
3. Modify DFS to cycle OIDs using the new function.

Required-githooks: true

Change-Id: I4b35ba1b3e2285f760d3be8db9e5bc724f24d772
Signed-off-by: Jeff Olivier <jeffolivier@google.com>

src/client/dfs/dfs_internal.h

@@ -99,9 +99,6 @@
 /** Max recursion depth for symlinks */
 #define DFS_MAX_RECURSION  40
 
-/** MAX value for the HI OID */
-#define MAX_OID_HI         ((1UL << 32) - 1)
-
 typedef uint64_t dfs_magic_t;
 typedef uint16_t dfs_sb_ver_t;
 typedef uint16_t dfs_layout_ver_t;
@@ -162,6 +159,8 @@ struct dfs {
 	daos_handle_t        coh;
 	/** refcount on cont handle that through the DFS API */
 	uint32_t             coh_refcount;
+	/** The last oid.hi in the sequence */
+	uint32_t             last_hi;
 	/** Object ID reserved for this DFS (see oid_gen below) */
 	daos_obj_id_t        oid;
 	/** superblock object OID */
@@ -337,20 +336,24 @@ oid_gen(dfs_t *dfs, daos_oclass_id_t oclass, bool file, daos_obj_id_t *oid)
 
 	D_MUTEX_LOCK(&dfs->lock);
 	/** If we ran out of local OIDs, alloc one from the container */
-	if (dfs->oid.hi >= MAX_OID_HI) {
+	if (dfs->oid.hi == dfs->last_hi) {
 		/** Allocate an OID for the namespace */
 		rc = daos_cont_alloc_oids(dfs->coh, 1, &dfs->oid.lo, NULL);
 		if (rc) {
 			D_ERROR("daos_cont_alloc_oids() Failed (%d)\n", rc);
 			D_MUTEX_UNLOCK(&dfs->lock);
 			return daos_der2errno(rc);
 		}
-		dfs->oid.hi = 0;
+		/** Start such that dfs->last_hi will be final value */
+		dfs->oid.hi = dfs->last_hi;
 	}
 
 	/** set oid and lo, bump the current hi value */
 	oid->lo = dfs->oid.lo;
-	oid->hi = dfs->oid.hi++;
+	daos_obj_oid_cycle(&dfs->oid);
+	if (unlikely(dfs->oid.lo == RESERVED_LO && dfs->oid.hi <= 1))
+		daos_obj_oid_cycle(&dfs->oid); /* Avoid reserved oids */
+	oid->hi = dfs->oid.hi;
 	D_MUTEX_UNLOCK(&dfs->lock);
 
 	/** if a regular file, use UINT64 typed dkeys for the array object */

src/client/dfs/mnt.c

@@ -683,20 +683,20 @@ dfs_mount(daos_handle_t poh, daos_handle_t coh, int flags, dfs_t **_dfs)
 
 	/** if RW, allocate an OID for the namespace */
 	if (amode == O_RDWR) {
+		dfs->last_hi = (unsigned int)d_rand();
+		/** Avoid potential conflict with SB or ROOT */
+		if (dfs->last_hi <= 1)
+			dfs->last_hi = 2;
+
 		rc = daos_cont_alloc_oids(coh, 1, &dfs->oid.lo, NULL);
 		if (rc) {
 			D_ERROR("daos_cont_alloc_oids() Failed, " DF_RC "\n", DP_RC(rc));
 			D_GOTO(err_root, rc = daos_der2errno(rc));
 		}
 
-		/*
-		 * if this is the first time we allocate on this container,
-		 * account 0 for SB, 1 for root obj.
-		 */
-		if (dfs->oid.lo == RESERVED_LO)
-			dfs->oid.hi = ROOT_HI + 1;
-		else
-			dfs->oid.hi = 0;
+		dfs->oid.hi = dfs->last_hi;
+		/** Increment so that dfs->last_hi is the last value */
+		daos_obj_oid_cycle(&dfs->oid);
 	}
 
 	dfs->mounted = DFS_MOUNT;
@@ -1016,7 +1016,7 @@ dfs_global2local(daos_handle_t poh, daos_handle_t coh, int flags, d_iov_t glob,
 
 	/** allocate a new oid on the next file or dir creation */
 	dfs->oid.lo = 0;
-	dfs->oid.hi = MAX_OID_HI;
+	dfs->oid.hi = dfs->last_hi;
 
 	rc = D_MUTEX_INIT(&dfs->lock, NULL);
 	if (rc != 0) {

src/include/daos_obj.h

@@ -1,5 +1,5 @@
 /**
- * (C) Copyright 2015-2023 Intel Corporation.
+ * (C) Copyright 2015-2024 Intel Corporation.
  *
  * SPDX-License-Identifier: BSD-2-Clause-Patent
  */
@@ -564,6 +564,20 @@ daos_obj_generate_oid(daos_handle_t coh, daos_obj_id_t *oid,
 		      enum daos_otype_t type, daos_oclass_id_t cid,
 		      daos_oclass_hints_t hints, uint32_t args);
 
+/**
+ * This function, if called 2^32 times will set oid->hi to every unique 32-bit
+ * value. The caller is responsible for setting the initial value, tracking the
+ * final value, and avoiding any values that are otherwise reserved.
+ *
+ * \param[in, out]	oid	oid to cycle
+ */
+static inline void
+daos_obj_oid_cycle(daos_obj_id_t *oid)
+{
+	/** Uses a large prime number to guarantee hitting every unique value */
+	oid->hi = (oid->hi + 999999937) & UINT_MAX;
+}
+
 /**
  * Open an DAOS object.
  *