diff --git a/src/basic/hashmap.c b/src/basic/hashmap.c
index b9efaafa3c..75119866d4 100644
--- a/src/basic/hashmap.c
+++ b/src/basic/hashmap.c
@@ -274,27 +274,31 @@ static _used_ const struct hashmap_type_info hashmap_type_info[_HASHMAP_TYPE_MAX
         },
 };
 
+void hashmap_trim_pools(void) {
+        int r;
+
+        /* The pool is only allocated by the main thread, but the memory can be passed to other
+         * threads. Let's clean up if we are the main thread and no other threads are live. */
+
+        /* We build our own is_main_thread() here, which doesn't use C11 TLS based caching of the
+         * result. That's because valgrind apparently doesn't like TLS to be used from a GCC destructor. */
+        if (getpid() != gettid())
+                return (void) log_debug("Not cleaning up memory pools, not in main thread.");
+
+        r = get_process_threads(0);
+        if (r < 0)
+                return (void) log_debug_errno(r, "Failed to determine number of threads, not cleaning up memory pools: %m");
+        if (r != 1)
+                return (void) log_debug("Not cleaning up memory pools, running in multi-threaded process.");
+
+        mempool_trim(&hashmap_pool);
+        mempool_trim(&ordered_hashmap_pool);
+}
+
 #if VALGRIND
 _destructor_ static void cleanup_pools(void) {
-        _cleanup_free_ char *t = NULL;
-
         /* Be nice to valgrind */
-
-        /* The pool is only allocated by the main thread, but the memory can
-         * be passed to other threads. Let's clean up if we are the main thread
-         * and no other threads are live. */
-        /* We build our own is_main_thread() here, which doesn't use C11
-         * TLS based caching of the result. That's because valgrind apparently
-         * doesn't like malloc() (which C11 TLS internally uses) to be called
-         * from a GCC destructors. */
-        if (getpid() != gettid())
-                return;
-
-        if (get_process_threads(0) != 1)
-                return;
-
-        mempool_drop(&hashmap_pool);
-        mempool_drop(&ordered_hashmap_pool);
+        hashmap_cleanup_pools();
 }
 #endif
 
diff --git a/src/basic/hashmap.h b/src/basic/hashmap.h
index ebb5a63eb4..68d9b81cf2 100644
--- a/src/basic/hashmap.h
+++ b/src/basic/hashmap.h
@@ -443,3 +443,5 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(OrderedHashmap*, ordered_hashmap_free_free_free);
 DEFINE_TRIVIAL_CLEANUP_FUNC(IteratedCache*, iterated_cache_free);
 
 #define _cleanup_iterated_cache_free_ _cleanup_(iterated_cache_freep)
+
+void hashmap_trim_pools(void);
diff --git a/src/basic/mempool.c b/src/basic/mempool.c
index fff23fdbac..fa319bffdb 100644
--- a/src/basic/mempool.c
+++ b/src/basic/mempool.c
@@ -3,6 +3,7 @@
 #include <stdint.h>
 #include <stdlib.h>
 
+#include "format-util.h"
 #include "macro.h"
 #include "memory-util.h"
 #include "mempool.h"
@@ -13,21 +14,26 @@ struct pool {
         size_t n_used;
 };
 
+static void* pool_ptr(struct pool *p) {
+        return ((uint8_t*) ASSERT_PTR(p)) + ALIGN(sizeof(struct pool));
+}
+
 void* mempool_alloc_tile(struct mempool *mp) {
         size_t i;
 
         /* When a tile is released we add it to the list and simply
          * place the next pointer at its offset 0. */
 
+        assert(mp);
         assert(mp->tile_size >= sizeof(void*));
         assert(mp->at_least > 0);
 
         if (mp->freelist) {
-                void *r;
+                void *t;
 
-                r = mp->freelist;
-                mp->freelist = * (void**) mp->freelist;
-                return r;
+                t = mp->freelist;
+                mp->freelist = *(void**) mp->freelist;
+                return t;
         }
 
         if (_unlikely_(!mp->first_pool) ||
@@ -53,7 +59,7 @@ void* mempool_alloc_tile(struct mempool *mp) {
 
         i = mp->first_pool->n_used++;
 
-        return ((uint8_t*) mp->first_pool) + ALIGN(sizeof(struct pool)) + i*mp->tile_size;
+        return (uint8_t*) pool_ptr(mp->first_pool) + i*mp->tile_size;
 }
 
 void* mempool_alloc0_tile(struct mempool *mp) {
@@ -65,19 +71,103 @@ void* mempool_alloc0_tile(struct mempool *mp) {
         return p;
 }
 
-void mempool_free_tile(struct mempool *mp, void *p) {
-        * (void**) p = mp->freelist;
+void* mempool_free_tile(struct mempool *mp, void *p) {
+        assert(mp);
+
+        if (!p)
+                return NULL;
+
+        *(void**) p = mp->freelist;
         mp->freelist = p;
+
+        return NULL;
 }
 
-#if VALGRIND
-void mempool_drop(struct mempool *mp) {
-        struct pool *p = mp->first_pool;
-        while (p) {
-                struct pool *n;
-                n = p->next;
-                free(p);
-                p = n;
+static bool pool_contains(struct mempool *mp, struct pool *p, void *ptr) {
+        size_t off;
+        void *a;
+
+        assert(mp);
+        assert(p);
+
+        if (!ptr)
+                return false;
+
+        a = pool_ptr(p);
+        if ((uint8_t*) ptr < (uint8_t*) a)
+                return false;
+
+        off = (uint8_t*) ptr - (uint8_t*) a;
+        assert(off % mp->tile_size == 0);
+
+        return off < mp->tile_size * p->n_tiles;
+}
+
+static bool pool_is_unused(struct mempool *mp, struct pool *p) {
+        assert(mp);
+        assert(p);
+
+        if (p->n_used == 0)
+                return true;
+
+        /* Check if all tiles in this specific pool are in the freelist. */
+        size_t n = 0;
+        void *i = mp->freelist;
+        while (i) {
+                if (pool_contains(mp, p, i))
+                        n++;
+
+                i = *(void**) i;
+        }
+
+        assert(n <= p->n_used);
+
+        return n == p->n_used;
+}
+
+static void pool_unlink(struct mempool *mp, struct pool *p) {
+        size_t m = 0;
+
+        assert(mp);
+        assert(p);
+
+        if (p->n_used == 0)
+                return;
+
+        void **i = &mp->freelist;
+        while (*i) {
+                void *d = *i;
+
+                if (pool_contains(mp, p, d)) {
+                        *i = *(void**) d;
+                        m++;
+
+                        if (m == p->n_used)
+                                break;
+                } else
+                        i = (void**) d;
         }
 }
-#endif
+
+void mempool_trim(struct mempool *mp) {
+        size_t trimmed = 0, left = 0;
+
+        assert(mp);
+
+        struct pool **p = &mp->first_pool;
+        while (*p) {
+                struct pool *d = *p;
+
+                if (pool_is_unused(mp, d)) {
+                        trimmed += d->n_tiles * mp->tile_size;
+                        pool_unlink(mp, d);
+                        *p = d->next;
+                        free(d);
+                } else {
+                        left += d->n_tiles * mp->tile_size;
+                        p = &d->next;
+                }
+        }
+
+        log_debug("Trimmed %s from memory pool %p. (%s left)", FORMAT_BYTES(trimmed), mp, FORMAT_BYTES(left));
+}
diff --git a/src/basic/mempool.h b/src/basic/mempool.h
index 539ccbdf06..ba588af451 100644
--- a/src/basic/mempool.h
+++ b/src/basic/mempool.h
@@ -10,12 +10,12 @@ struct mempool {
         struct pool *first_pool;
         void *freelist;
         size_t tile_size;
-        unsigned at_least;
+        size_t at_least;
 };
 
 void* mempool_alloc_tile(struct mempool *mp);
 void* mempool_alloc0_tile(struct mempool *mp);
-void mempool_free_tile(struct mempool *mp, void *p);
+void* mempool_free_tile(struct mempool *mp, void *p);
 
 #define DEFINE_MEMPOOL(pool_name, tile_type, alloc_at_least) \
 static struct mempool pool_name = { \
@@ -25,6 +25,4 @@ static struct mempool pool_name = { \
 
 __attribute__((weak)) bool mempool_enabled(void);
 
-#if VALGRIND
-void mempool_drop(struct mempool *mp);
-#endif
+void mempool_trim(struct mempool *mp);
diff --git a/src/test/meson.build b/src/test/meson.build
index f05e6a5694..1dbe12dc40 100644
--- a/src/test/meson.build
+++ b/src/test/meson.build
@@ -509,6 +509,8 @@ tests += [
 
         [files('test-tpm2.c')],
 
+        [files('test-mempool.c')],
+
         [files('test-replace-var.c')],
 
         [files('test-calendarspec.c')],
diff --git a/src/test/test-mempool.c b/src/test/test-mempool.c
new file mode 100644
index 0000000000..d3bc173fd4
--- /dev/null
+++ b/src/test/test-mempool.c
@@ -0,0 +1,92 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "mempool.h"
+#include "random-util.h"
+#include "tests.h"
+
+struct element {
+        uint64_t value;
+};
+
+DEFINE_MEMPOOL(test_mempool, struct element, 8);
+
+TEST(mempool_trim) {
+
+#define NN 4000
+        struct element *a[NN];
+        size_t n_freed = 0;
+
+        assert_se(!test_mempool.first_pool);
+        assert_se(!test_mempool.freelist);
+
+        mempool_trim(&test_mempool);
+
+        for (size_t i = 0; i < NN; i++) {
+                assert_se(a[i] = mempool_alloc_tile(&test_mempool));
+                a[i]->value = i;
+        }
+
+        mempool_trim(&test_mempool);
+
+        /* free up to one third randomly */
+        size_t x = 0;
+        for (size_t i = 0; i < NN/3; i++) {
+                x = (x + random_u64()) % ELEMENTSOF(a);
+                assert_se(!a[x] || a[x]->value == x);
+
+                if (a[x])
+                        n_freed ++;
+
+                a[x] = mempool_free_tile(&test_mempool, a[x]);
+        }
+
+        mempool_trim(&test_mempool);
+
+        /* free definitely at least one third */
+        for (size_t i = 2; i < NN; i += 3) {
+                assert_se(!a[i] || a[i]->value == i);
+                if (a[i])
+                        n_freed ++;
+                a[i] = mempool_free_tile(&test_mempool, a[i]);
+        }
+
+        mempool_trim(&test_mempool);
+
+        /* Allocate another set of tiles, which will fill up the free list and allocate some new tiles */
+        struct element *b[NN];
+        for (size_t i = 0; i < NN; i++) {
+                assert_se(b[i] = mempool_alloc_tile(&test_mempool));
+                b[i]->value = ~(uint64_t) i;
+        }
+
+        mempool_trim(&test_mempool);
+
+        /* free everything from the original set*/
+
+        for (size_t i = 0; i < NN; i += 1) {
+                assert_se(!a[i] || a[i]->value == i);
+                if (a[i])
+                        n_freed ++;
+                a[i] = mempool_free_tile(&test_mempool, a[i]);
+        }
+
+        mempool_trim(&test_mempool);
+
+        /* and now everything from the second set too */
+
+        for (size_t i = 0; i < NN; i += 1) {
+                assert_se(!b[i] || b[i]->value == ~(uint64_t) i);
+                if (b[i])
+                        n_freed ++;
+                b[i] = mempool_free_tile(&test_mempool, b[i]);
+        }
+
+        assert_se(n_freed == NN * 2);
+
+        mempool_trim(&test_mempool);
+
+        assert_se(!test_mempool.first_pool);
+        assert_se(!test_mempool.freelist);
+}
+
+DEFINE_TEST_MAIN(LOG_DEBUG);