cache garbage collector

Experimental code for cache GC.

I've set the defaults to values that seemed to work well (clear about ~10% of cache when 80% full, use 100 operations per transaction). Higher values for allowed cache usage didn't seem to properly prevent complete cache flush. High values of ops per transaction (or unlimited) caused issues as well.

Closes #257 (closed)

utils/kr_cache_gc/kr_cache_gc.c

 // standard includes
+#include <inttypes.h>
+#include <limits.h>
 #include <stdio.h>
 #include <time.h>
 #include <sys/stat.h>
@@ -15,7 +17,7 @@
 #include "kr_cache_gc.h"
 
 // TODO remove and use time(NULL) ! this is just for debug with pre-generated cache
-int64_t now = 1523701784;
+int64_t now = 1524301784;
 
 // section: timer
 // TODO replace/move to contrib
@@ -207,6 +209,7 @@ int kr_cache_gc(kr_cache_gc_cfg_t *cfg)
 	size_t cache_records = 0, deleted_records = 0;
 	size_t oversize_records = 0, already_gone = 0;;
 	size_t used_space = 0, rw_txn_count = 1;
+	int64_t min_expire = INT64_MAX;
 
 	while (it != NULL) {
 		knot_db_val_t key = { 0 }, val = { 0 };
@@ -222,6 +225,9 @@ int kr_cache_gc(kr_cache_gc_cfg_t *cfg)
 			struct entry_h *entry = entry_h_consistent(val, *entry_type);
 			int64_t over = entry->time + entry->ttl;
 			over -= now;
+			if (over < min_expire) {
+				min_expire = over;
+			}
 			if (over < 0) {
 				knot_db_val_t *todelete;
 				if ((cfg->temp_keys_space > 0 &&
@@ -242,6 +248,9 @@ int kr_cache_gc(kr_cache_gc_cfg_t *cfg)
 
 	printf("Cache analyzed in %.2lf secs, %zu records types", gc_timer_end(&timer_analyze), cache_records);
 	rrtypelist_print(&cache_rrtypes);
+	if (min_expire < INT64_MAX) {
+		printf("Minimum expire in %"PRId64" secs\n", min_expire);
+	}
 	printf("%zu records to be deleted using %.2lf MBytes of temporary memory, %zu records skipped due to memory limit.\n",
 	       to_del.size, ((double)used_space / 1048576.0), oversize_records);
 	rrtype_dynarray_free(&cache_rrtypes);