1 /* 2 * CDDL HEADER START 3 * 4 * This file and its contents are supplied under the terms of the 5 * Common Development and Distribution License ("CDDL"), version 1.0. 6 * You may only use this file in accordance with the terms of version 7 * 1.0 of the CDDL. 8 * 9 * A full copy of the text of the CDDL should have accompanied this 10 * source. A copy of the CDDL is also available via the Internet at 11 * http://www.illumos.org/license/CDDL. 12 * 13 * CDDL HEADER END 14 */ 15 /* 16 * Copyright (c) 2017, 2018 by Delphix. All rights reserved. 17 */ 18 19 #include <sys/zfs_context.h> 20 #include <sys/aggsum.h> 21 22 /* 23 * Aggregate-sum counters are a form of fanned-out counter, used when atomic 24 * instructions on a single field cause enough CPU cache line contention to 25 * slow system performance. Due to their increased overhead and the expense 26 * involved with precisely reading from them, they should only be used in cases 27 * where the write rate (increment/decrement) is much higher than the read rate 28 * (get value). 29 * 30 * Aggregate sum counters are comprised of two basic parts, the core and the 31 * buckets. The core counter contains a lock for the entire counter, as well 32 * as the current upper and lower bounds on the value of the counter. The 33 * aggsum_bucket structure contains a per-bucket lock to protect the contents of 34 * the bucket, the current amount that this bucket has changed from the global 35 * counter (called the delta), and the amount of increment and decrement we have 36 * "borrowed" from the core counter. 37 * 38 * The basic operation of an aggsum is simple. Threads that wish to modify the 39 * counter will modify one bucket's counter (determined by their current CPU, to 40 * help minimize lock and cache contention). If the bucket already has 41 * sufficient capacity borrowed from the core structure to handle their request, 42 * they simply modify the delta and return. If the bucket does not, we clear 43 * the bucket's current state (to prevent the borrowed amounts from getting too 44 * large), and borrow more from the core counter. Borrowing is done by adding to 45 * the upper bound (or subtracting from the lower bound) of the core counter, 46 * and setting the borrow value for the bucket to the amount added (or 47 * subtracted). Clearing the bucket is the opposite; we add the current delta 48 * to both the lower and upper bounds of the core counter, subtract the borrowed 49 * incremental from the upper bound, and add the borrowed decrement from the 50 * lower bound. Note that only borrowing and clearing require access to the 51 * core counter; since all other operations access CPU-local resources, 52 * performance can be much higher than a traditional counter. 53 * 54 * Threads that wish to read from the counter have a slightly more challenging 55 * task. It is fast to determine the upper and lower bounds of the aggum; this 56 * does not require grabbing any locks. This suffices for cases where an 57 * approximation of the aggsum's value is acceptable. However, if one needs to 58 * know whether some specific value is above or below the current value in the 59 * aggsum, they invoke aggsum_compare(). This function operates by repeatedly 60 * comparing the target value to the upper and lower bounds of the aggsum, and 61 * then clearing a bucket. This proceeds until the target is outside of the 62 * upper and lower bounds and we return a response, or the last bucket has been 63 * cleared and we know that the target is equal to the aggsum's value. Finally, 64 * the most expensive operation is determining the precise value of the aggsum. 65 * To do this, we clear every bucket and then return the upper bound (which must 66 * be equal to the lower bound). What makes aggsum_compare() and aggsum_value() 67 * expensive is clearing buckets. This involves grabbing the global lock 68 * (serializing against themselves and borrow operations), grabbing a bucket's 69 * lock (preventing threads on those CPUs from modifying their delta), and 70 * zeroing out the borrowed value (forcing that thread to borrow on its next 71 * request, which will also be expensive). This is what makes aggsums well 72 * suited for write-many read-rarely operations. 73 * 74 * Note that the aggsums do not expand if more CPUs are hot-added. In that 75 * case, we will have less fanout than boot_ncpus, but we don't want to always 76 * reserve the RAM necessary to create the extra slots for additional CPUs up 77 * front, and dynamically adding them is a complex task. 78 */ 79 80 /* 81 * We will borrow 2^aggsum_borrow_shift times the current request, so we will 82 * have to get the as_lock approximately every 2^aggsum_borrow_shift calls to 83 * aggsum_add(). 84 */ 85 static uint_t aggsum_borrow_shift = 4; 86 87 void 88 aggsum_init(aggsum_t *as, uint64_t value) 89 { 90 memset(as, 0, sizeof (*as)); 91 as->as_lower_bound = as->as_upper_bound = value; 92 mutex_init(&as->as_lock, NULL, MUTEX_DEFAULT, NULL); 93 /* 94 * Too many buckets may hurt read performance without improving 95 * write. From 12 CPUs use bucket per 2 CPUs, from 48 per 4, etc. 96 */ 97 as->as_bucketshift = highbit64(boot_ncpus / 6) / 2; 98 as->as_numbuckets = ((boot_ncpus - 1) >> as->as_bucketshift) + 1; 99 as->as_buckets = kmem_zalloc(as->as_numbuckets * 100 sizeof (aggsum_bucket_t), KM_SLEEP); 101 for (int i = 0; i < as->as_numbuckets; i++) { 102 mutex_init(&as->as_buckets[i].asc_lock, 103 NULL, MUTEX_DEFAULT, NULL); 104 } 105 } 106 107 void 108 aggsum_fini(aggsum_t *as) 109 { 110 for (int i = 0; i < as->as_numbuckets; i++) 111 mutex_destroy(&as->as_buckets[i].asc_lock); 112 kmem_free(as->as_buckets, as->as_numbuckets * sizeof (aggsum_bucket_t)); 113 mutex_destroy(&as->as_lock); 114 } 115 116 int64_t 117 aggsum_lower_bound(aggsum_t *as) 118 { 119 return (atomic_load_64((volatile uint64_t *)&as->as_lower_bound)); 120 } 121 122 uint64_t 123 aggsum_upper_bound(aggsum_t *as) 124 { 125 return (atomic_load_64(&as->as_upper_bound)); 126 } 127 128 uint64_t 129 aggsum_value(aggsum_t *as) 130 { 131 int64_t lb; 132 uint64_t ub; 133 134 mutex_enter(&as->as_lock); 135 lb = as->as_lower_bound; 136 ub = as->as_upper_bound; 137 if (lb == ub) { 138 for (int i = 0; i < as->as_numbuckets; i++) { 139 ASSERT0(as->as_buckets[i].asc_delta); 140 ASSERT0(as->as_buckets[i].asc_borrowed); 141 } 142 mutex_exit(&as->as_lock); 143 return (lb); 144 } 145 for (int i = 0; i < as->as_numbuckets; i++) { 146 struct aggsum_bucket *asb = &as->as_buckets[i]; 147 if (asb->asc_borrowed == 0) 148 continue; 149 mutex_enter(&asb->asc_lock); 150 lb += asb->asc_delta + asb->asc_borrowed; 151 ub += asb->asc_delta - asb->asc_borrowed; 152 asb->asc_delta = 0; 153 asb->asc_borrowed = 0; 154 mutex_exit(&asb->asc_lock); 155 } 156 ASSERT3U(lb, ==, ub); 157 atomic_store_64((volatile uint64_t *)&as->as_lower_bound, lb); 158 atomic_store_64(&as->as_upper_bound, lb); 159 mutex_exit(&as->as_lock); 160 161 return (lb); 162 } 163 164 void 165 aggsum_add(aggsum_t *as, int64_t delta) 166 { 167 struct aggsum_bucket *asb; 168 int64_t borrow; 169 170 asb = &as->as_buckets[(CPU_SEQID_UNSTABLE >> as->as_bucketshift) % 171 as->as_numbuckets]; 172 173 /* Try fast path if we already borrowed enough before. */ 174 mutex_enter(&asb->asc_lock); 175 if (asb->asc_delta + delta <= (int64_t)asb->asc_borrowed && 176 asb->asc_delta + delta >= -(int64_t)asb->asc_borrowed) { 177 asb->asc_delta += delta; 178 mutex_exit(&asb->asc_lock); 179 return; 180 } 181 mutex_exit(&asb->asc_lock); 182 183 /* 184 * We haven't borrowed enough. Take the global lock and borrow 185 * considering what is requested now and what we borrowed before. 186 */ 187 borrow = (delta < 0 ? -delta : delta); 188 borrow <<= aggsum_borrow_shift + as->as_bucketshift; 189 mutex_enter(&as->as_lock); 190 if (borrow >= asb->asc_borrowed) 191 borrow -= asb->asc_borrowed; 192 else 193 borrow = (borrow - (int64_t)asb->asc_borrowed) / 4; 194 mutex_enter(&asb->asc_lock); 195 delta += asb->asc_delta; 196 asb->asc_delta = 0; 197 asb->asc_borrowed += borrow; 198 mutex_exit(&asb->asc_lock); 199 atomic_store_64((volatile uint64_t *)&as->as_lower_bound, 200 as->as_lower_bound + delta - borrow); 201 atomic_store_64(&as->as_upper_bound, 202 as->as_upper_bound + delta + borrow); 203 mutex_exit(&as->as_lock); 204 } 205 206 /* 207 * Compare the aggsum value to target efficiently. Returns -1 if the value 208 * represented by the aggsum is less than target, 1 if it's greater, and 0 if 209 * they are equal. 210 */ 211 int 212 aggsum_compare(aggsum_t *as, uint64_t target) 213 { 214 int64_t lb; 215 uint64_t ub; 216 int i; 217 218 if (atomic_load_64(&as->as_upper_bound) < target) 219 return (-1); 220 lb = atomic_load_64((volatile uint64_t *)&as->as_lower_bound); 221 if (lb > 0 && (uint64_t)lb > target) 222 return (1); 223 mutex_enter(&as->as_lock); 224 lb = as->as_lower_bound; 225 ub = as->as_upper_bound; 226 for (i = 0; i < as->as_numbuckets; i++) { 227 struct aggsum_bucket *asb = &as->as_buckets[i]; 228 if (asb->asc_borrowed == 0) 229 continue; 230 mutex_enter(&asb->asc_lock); 231 lb += asb->asc_delta + asb->asc_borrowed; 232 ub += asb->asc_delta - asb->asc_borrowed; 233 asb->asc_delta = 0; 234 asb->asc_borrowed = 0; 235 mutex_exit(&asb->asc_lock); 236 if (ub < target || (lb > 0 && (uint64_t)lb > target)) 237 break; 238 } 239 if (i >= as->as_numbuckets) 240 ASSERT3U(lb, ==, ub); 241 atomic_store_64((volatile uint64_t *)&as->as_lower_bound, lb); 242 atomic_store_64(&as->as_upper_bound, ub); 243 mutex_exit(&as->as_lock); 244 return (ub < target ? -1 : (uint64_t)lb > target ? 1 : 0); 245 } 246