Allow PAI to calculate time until deferred work

Previously the calculation of sleep time between wakeups was implemented within
background_thread. This resulted in some parts of decay and hpa specific
logic mixing with background thread implementation. In this change, background
thread delegates this calculation to arena and it, in turn, delegates it to PAI.
The next step is to implement the actual calculation of time until deferred work
in HPA.

test/unit/decay.c

@@ -36,6 +36,37 @@ TEST_BEGIN(test_decay_ms_valid) {
 }
 TEST_END
 
+TEST_BEGIN(test_decay_npages_purge_in) {
+	decay_t decay;
+	memset(&decay, 0, sizeof(decay));
+
+	nstime_t curtime;
+	nstime_init(&curtime, 0);
+
+	uint64_t decay_ms = 1000;
+	nstime_t decay_nstime;
+	nstime_init(&decay_nstime, decay_ms * 1000 * 1000);
+	expect_false(decay_init(&decay, &curtime, (ssize_t)decay_ms),
+	    "Failed to initialize decay");
+
+	const size_t new_pages = 100;
+
+	nstime_t time;
+	nstime_copy(&time, &decay_nstime);
+	expect_u64_eq(decay_npages_purge_in(&decay, &time, new_pages),
+	    new_pages, "Not all pages are expected to decay in decay_ms");
+
+	nstime_init(&time, 0);
+	expect_u64_eq(decay_npages_purge_in(&decay, &time, new_pages), 0,
+	    "More than zero pages are expected to instantly decay");
+
+	nstime_copy(&time, &decay_nstime);
+	nstime_idivide(&time, 2);
+	expect_u64_eq(decay_npages_purge_in(&decay, &time, new_pages),
+	    new_pages / 2, "Not half of pages decay in half the decay period");
+}
+TEST_END
+
 TEST_BEGIN(test_decay_maybe_advance_epoch) {
 	decay_t decay;
 	memset(&decay, 0, sizeof(decay));
@@ -244,6 +275,7 @@ main(void) {
 	return test(
 	    test_decay_init,
 	    test_decay_ms_valid,
+	    test_decay_npages_purge_in,
 	    test_decay_maybe_advance_epoch,
 	    test_decay_empty,
 	    test_decay,