usr/src/uts/common/fs/zfs/aggsum.c

   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 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
  75 /*
  76  * We will borrow aggsum_borrow_multiplier times the current request, so we will
  77  * have to get the as_lock approximately every aggsum_borrow_multiplier calls to
  78  * aggsum_delta().
  79  */
  80 static uint_t aggsum_borrow_multiplier = 10;
  81
  82 void
  83 aggsum_init(aggsum_t *as, uint64_t value)
  84 {
  85         bzero(as, sizeof (*as));
  86         as->as_lower_bound = as->as_upper_bound = value;
  87         mutex_init(&as->as_lock, NULL, MUTEX_DEFAULT, NULL);
  88         as->as_numbuckets = boot_ncpus;
  89         as->as_buckets = kmem_zalloc(boot_ncpus * sizeof (aggsum_bucket_t),
  90             KM_SLEEP);
  91         for (int i = 0; i < as->as_numbuckets; i++) {
  92                 mutex_init(&as->as_buckets[i].asc_lock,
  93                     NULL, MUTEX_DEFAULT, NULL);
  94         }
  95 }
  96
  97 void
  98 aggsum_fini(aggsum_t *as)
  99 {
 100         for (int i = 0; i < as->as_numbuckets; i++)
 101                 mutex_destroy(&as->as_buckets[i].asc_lock);
 102         mutex_destroy(&as->as_lock);
 103 }
 104
 105 int64_t
 106 aggsum_lower_bound(aggsum_t *as)
 107 {
 108         return (as->as_lower_bound);
 109 }
 110
 111 int64_t
 112 aggsum_upper_bound(aggsum_t *as)
 113 {
 114         return (as->as_upper_bound);
 115 }
 116
 117 static void
 118 aggsum_flush_bucket(aggsum_t *as, struct aggsum_bucket *asb)
 119 {
 120         ASSERT(MUTEX_HELD(&as->as_lock));
 121         ASSERT(MUTEX_HELD(&asb->asc_lock));
 122
 123         /*
 124          * We use atomic instructions for this because we read the upper and
 125          * lower bounds without the lock, so we need stores to be atomic.
 126          */
 127         atomic_add_64((volatile uint64_t *)&as->as_lower_bound, asb->asc_delta);
 128         atomic_add_64((volatile uint64_t *)&as->as_upper_bound, asb->asc_delta);
 129         asb->asc_delta = 0;
 130         atomic_add_64((volatile uint64_t *)&as->as_upper_bound,
 131             -asb->asc_borrowed);
 132         atomic_add_64((volatile uint64_t *)&as->as_lower_bound,
 133             asb->asc_borrowed);
 134         asb->asc_borrowed = 0;
 135 }
 136
 137 uint64_t
 138 aggsum_value(aggsum_t *as)
 139 {
 140         int64_t rv;
 141
 142         mutex_enter(&as->as_lock);
 143         if (as->as_lower_bound == as->as_upper_bound) {
 144                 rv = as->as_lower_bound;
 145                 for (int i = 0; i < as->as_numbuckets; i++) {
 146                         ASSERT0(as->as_buckets[i].asc_delta);
 147                         ASSERT0(as->as_buckets[i].asc_borrowed);
 148                 }
 149                 mutex_exit(&as->as_lock);
 150                 return (rv);
 151         }
 152         for (int i = 0; i < as->as_numbuckets; i++) {
 153                 struct aggsum_bucket *asb = &as->as_buckets[i];
 154                 mutex_enter(&asb->asc_lock);
 155                 aggsum_flush_bucket(as, asb);
 156                 mutex_exit(&asb->asc_lock);
 157         }
 158         VERIFY3U(as->as_lower_bound, ==, as->as_upper_bound);
 159         rv = as->as_lower_bound;
 160         mutex_exit(&as->as_lock);
 161
 162         return (rv);
 163 }
 164
 165 static void
 166 aggsum_borrow(aggsum_t *as, int64_t delta, struct aggsum_bucket *asb)
 167 {
 168         int64_t abs_delta = (delta < 0 ? -delta : delta);
 169         mutex_enter(&as->as_lock);
 170         mutex_enter(&asb->asc_lock);
 171
 172         aggsum_flush_bucket(as, asb);
 173
 174         atomic_add_64((volatile uint64_t *)&as->as_upper_bound, abs_delta);
 175         atomic_add_64((volatile uint64_t *)&as->as_lower_bound, -abs_delta);
 176         asb->asc_borrowed = abs_delta;
 177
 178         mutex_exit(&asb->asc_lock);
 179         mutex_exit(&as->as_lock);
 180 }
 181
 182 void
 183 aggsum_add(aggsum_t *as, int64_t delta)
 184 {
 185         struct aggsum_bucket *asb =
 186             &as->as_buckets[CPU_SEQID % as->as_numbuckets];
 187
 188         for (;;) {
 189                 mutex_enter(&asb->asc_lock);
 190                 if (asb->asc_delta + delta <= (int64_t)asb->asc_borrowed &&
 191                     asb->asc_delta + delta >= -(int64_t)asb->asc_borrowed) {
 192                         asb->asc_delta += delta;
 193                         mutex_exit(&asb->asc_lock);
 194                         return;
 195                 }
 196                 mutex_exit(&asb->asc_lock);
 197                 aggsum_borrow(as, delta * aggsum_borrow_multiplier, asb);
 198         }
 199 }
 200
 201 /*
 202  * Compare the aggsum value to target efficiently. Returns -1 if the value
 203  * represented by the aggsum is less than target, 1 if it's greater, and 0 if
 204  * they are equal.
 205  */
 206 int
 207 aggsum_compare(aggsum_t *as, uint64_t target)
 208 {
 209         if (as->as_upper_bound < target)
 210                 return (-1);
 211         if (as->as_lower_bound > target)
 212                 return (1);
 213         mutex_enter(&as->as_lock);
 214         for (int i = 0; i < as->as_numbuckets; i++) {
 215                 struct aggsum_bucket *asb = &as->as_buckets[i];
 216                 mutex_enter(&asb->asc_lock);
 217                 aggsum_flush_bucket(as, asb);
 218                 mutex_exit(&asb->asc_lock);
 219                 if (as->as_upper_bound < target) {
 220                         mutex_exit(&as->as_lock);
 221                         return (-1);
 222                 }
 223                 if (as->as_lower_bound > target) {
 224                         mutex_exit(&as->as_lock);
 225                         return (1);
 226                 }
 227         }
 228         VERIFY3U(as->as_lower_bound, ==, as->as_upper_bound);
 229         ASSERT3U(as->as_lower_bound, ==, target);
 230         mutex_exit(&as->as_lock);
 231         return (0);
 232 }