Tcache: Add tcache gc delay option.

This can reduce flushing frequency for small size classes.

include/jemalloc/internal/tcache_externs.h

@@ -9,6 +9,7 @@ extern unsigned opt_tcache_nslots_small_max;
 extern unsigned opt_tcache_nslots_large;
 extern ssize_t opt_lg_tcache_shift;
 extern size_t opt_tcache_gc_incr_bytes;
+extern size_t opt_tcache_gc_delay_bytes;
 
 /*
  * Number of tcache bins.  There are SC_NBINS small-object bins, plus 0 or more

include/jemalloc/internal/tcache_structs.h

@@ -36,6 +36,11 @@ struct tcache_slow_s {
 	uint8_t		lg_fill_div[SC_NBINS];
 	/* For small bins, whether has been refilled since last GC. */
 	bool		bin_refilled[SC_NBINS];
+	/*
+	 * For small bins, the number of items we can pretend to flush before
+	 * actually flushing.
+	 */
+	uint8_t		bin_flush_delay_items[SC_NBINS];
 	/*
 	 * The start of the allocation containing the dynamic allocation for
 	 * either the cache bins alone, or the cache bin memory as well as this