diff --git a/.gitignore b/.gitignore
index 6669bf0c6c..0a1f93b25a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -190,6 +190,7 @@
 /test-mktemp
 /test-parse-options
 /test-path-utils
+/test-read-cache
 /test-regex
 /test-revision-walking
 /test-run-command
diff --git a/Makefile b/Makefile
index 14a38cc422..3c17234e19 100644
--- a/Makefile
+++ b/Makefile
@@ -565,6 +565,7 @@ TEST_PROGRAMS_NEED_X += test-mergesort
 TEST_PROGRAMS_NEED_X += test-mktemp
 TEST_PROGRAMS_NEED_X += test-parse-options
 TEST_PROGRAMS_NEED_X += test-path-utils
+TEST_PROGRAMS_NEED_X += test-read-cache
 TEST_PROGRAMS_NEED_X += test-regex
 TEST_PROGRAMS_NEED_X += test-revision-walking
 TEST_PROGRAMS_NEED_X += test-run-command
diff --git a/read-cache.c b/read-cache.c
index 7040e7962b..d97e3bf7cd 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1518,8 +1518,9 @@ int discard_index(struct index_state *istate)
 	free_name_hash(istate);
 	cache_tree_free(&(istate->cache_tree));
 	istate->initialized = 0;
-
-	/* no need to throw away allocated active_cache */
+	free(istate->cache);
+	istate->cache = NULL;
+	istate->cache_alloc = 0;
 	return 0;
 }
 
diff --git a/t/perf/p0002-read-cache.sh b/t/perf/p0002-read-cache.sh
new file mode 100755
index 0000000000..9180ae9343
--- /dev/null
+++ b/t/perf/p0002-read-cache.sh
@@ -0,0 +1,14 @@
+#!/bin/sh
+
+test_description="Tests performance of reading the index"
+
+. ./perf-lib.sh
+
+test_perf_default_repo
+
+count=1000
+test_perf "read_cache/discard_cache $count times" "
+	test-read-cache $count
+"
+
+test_done
diff --git a/test-read-cache.c b/test-read-cache.c
new file mode 100644
index 0000000000..b25bcf139b
--- /dev/null
+++ b/test-read-cache.c
@@ -0,0 +1,13 @@
+#include "cache.h"
+
+int main (int argc, char **argv)
+{
+	int i, cnt = 1;
+	if (argc == 2)
+		cnt = strtol(argv[1], NULL, 0);
+	for (i = 0; i < cnt; i++) {
+		read_cache();
+		discard_cache();
+	}
+	return 0;
+}