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
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.
16 * Copyright (c) 2017, 2018 by Delphix. All rights reserved.
19 #include <sys/zfs_context.h>
20 #include <sys/aggsum.h>
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
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.
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.
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.
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
80 static uint_t aggsum_borrow_multiplier = 10;
83 aggsum_init(aggsum_t *as, uint64_t value)
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),
91 for (int i = 0; i < as->as_numbuckets; i++) {
92 mutex_init(&as->as_buckets[i].asc_lock,
93 NULL, MUTEX_DEFAULT, NULL);
98 aggsum_fini(aggsum_t *as)
100 for (int i = 0; i < as->as_numbuckets; i++)
101 mutex_destroy(&as->as_buckets[i].asc_lock);
102 kmem_free(as->as_buckets, as->as_numbuckets * sizeof (aggsum_bucket_t));
103 mutex_destroy(&as->as_lock);
107 aggsum_lower_bound(aggsum_t *as)
109 return (as->as_lower_bound);
113 aggsum_upper_bound(aggsum_t *as)
115 return (as->as_upper_bound);
119 aggsum_flush_bucket(aggsum_t *as, struct aggsum_bucket *asb)
121 ASSERT(MUTEX_HELD(&as->as_lock));
122 ASSERT(MUTEX_HELD(&asb->asc_lock));
125 * We use atomic instructions for this because we read the upper and
126 * lower bounds without the lock, so we need stores to be atomic.
128 atomic_add_64((volatile uint64_t *)&as->as_lower_bound,
129 asb->asc_delta + asb->asc_borrowed);
130 atomic_add_64((volatile uint64_t *)&as->as_upper_bound,
131 asb->asc_delta - asb->asc_borrowed);
133 asb->asc_borrowed = 0;
137 aggsum_value(aggsum_t *as)
141 mutex_enter(&as->as_lock);
142 if (as->as_lower_bound == as->as_upper_bound) {
143 rv = as->as_lower_bound;
144 for (int i = 0; i < as->as_numbuckets; i++) {
145 ASSERT0(as->as_buckets[i].asc_delta);
146 ASSERT0(as->as_buckets[i].asc_borrowed);
148 mutex_exit(&as->as_lock);
151 for (int i = 0; i < as->as_numbuckets; i++) {
152 struct aggsum_bucket *asb = &as->as_buckets[i];
153 mutex_enter(&asb->asc_lock);
154 aggsum_flush_bucket(as, asb);
155 mutex_exit(&asb->asc_lock);
157 VERIFY3U(as->as_lower_bound, ==, as->as_upper_bound);
158 rv = as->as_lower_bound;
159 mutex_exit(&as->as_lock);
165 aggsum_add(aggsum_t *as, int64_t delta)
167 struct aggsum_bucket *asb =
168 &as->as_buckets[CPU_SEQID % as->as_numbuckets];
171 /* Try fast path if we already borrowed enough before. */
172 mutex_enter(&asb->asc_lock);
173 if (asb->asc_delta + delta <= (int64_t)asb->asc_borrowed &&
174 asb->asc_delta + delta >= -(int64_t)asb->asc_borrowed) {
175 asb->asc_delta += delta;
176 mutex_exit(&asb->asc_lock);
179 mutex_exit(&asb->asc_lock);
182 * We haven't borrowed enough. Take the global lock and borrow
183 * considering what is requested now and what we borrowed before.
185 borrow = (delta < 0 ? -delta : delta) * aggsum_borrow_multiplier;
186 mutex_enter(&as->as_lock);
187 mutex_enter(&asb->asc_lock);
188 delta += asb->asc_delta;
190 if (borrow >= asb->asc_borrowed)
191 borrow -= asb->asc_borrowed;
193 borrow = (borrow - (int64_t)asb->asc_borrowed) / 4;
194 asb->asc_borrowed += borrow;
195 atomic_add_64((volatile uint64_t *)&as->as_lower_bound,
197 atomic_add_64((volatile uint64_t *)&as->as_upper_bound,
199 mutex_exit(&asb->asc_lock);
200 mutex_exit(&as->as_lock);
204 * Compare the aggsum value to target efficiently. Returns -1 if the value
205 * represented by the aggsum is less than target, 1 if it's greater, and 0 if
209 aggsum_compare(aggsum_t *as, uint64_t target)
211 if (as->as_upper_bound < target)
213 if (as->as_lower_bound > target)
215 mutex_enter(&as->as_lock);
216 for (int i = 0; i < as->as_numbuckets; i++) {
217 struct aggsum_bucket *asb = &as->as_buckets[i];
218 mutex_enter(&asb->asc_lock);
219 aggsum_flush_bucket(as, asb);
220 mutex_exit(&asb->asc_lock);
221 if (as->as_upper_bound < target) {
222 mutex_exit(&as->as_lock);
225 if (as->as_lower_bound > target) {
226 mutex_exit(&as->as_lock);
230 VERIFY3U(as->as_lower_bound, ==, as->as_upper_bound);
231 ASSERT3U(as->as_lower_bound, ==, target);
232 mutex_exit(&as->as_lock);