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40 lines
2.4 KiB
C
40 lines
2.4 KiB
C
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#ifndef THREAD_LOCAL_COUNTER_H
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#define THREAD_LOCAL_COUNTER_H
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// Overview: A partitioned_counter provides a counter that can be incremented and the running sum can be read at any time.
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// We assume that increments are frequent, whereas reading is infrequent.
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// Implementation hint: Use thread-local storage so each thread increments its own data. The increment does not require a lock or atomic operation.
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// Reading the data can be performed by iterating over the thread-local versions, summing them up.
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// The data structure also includes a sum for all the threads that have died.
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// Use a pthread_key to create the thread-local versions. When a thread finishes, the system calls pthread_key destructor which can add that thread's copy
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// into the sum_of_dead counter.
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// Rationale: For statistics such as are found in engine status, we need a counter that requires no cache misses to increment. We've seen significant
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// performance speedups by removing certain counters. Rather than removing those statistics, we would like to just make the counter fast.
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// We generally increment the counters frequently, and want to fetch the values infrequently.
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// The counters are monotonic.
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// The counters can be split into many counters, which can be summed up at the end.
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// We don't care if we get slightly out-of-date counter sums when we read the counter. We don't care if there is a race on reading the a counter
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// variable and incrementing.
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// See tests/test_partitioned_counter.c for some performance measurements.
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// Operations:
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// create_partitioned_counter Create a counter initialized to zero.
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// destroy_partitioned_counter Destroy it.
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// increment_partitioned_counter Increment it. This is the frequent operation.
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// read_partitioned_counter Get the current value. This is infrequent.
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typedef struct partitioned_counter *PARTITIONED_COUNTER;
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PARTITIONED_COUNTER create_partitioned_counter(void);
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// Effect: Create a counter, initialized to zero.
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void destroy_partitioned_counter (PARTITIONED_COUNTER);
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// Effect: Destroy the counter. No operations on that counter are permitted after this.
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void increment_partitioned_counter (PARTITIONED_COUNTER, unsigned long amount);
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// Effect: Increment the counter by amount.
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// Requires: No overflows. This is a 64-bit unsigned counter.
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unsigned long read_partitioned_counter (PARTITIONED_COUNTER);
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// Effect: Return the current value of the counter.
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#endif
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