journald: keep statistics on how of we hit/miss the mmap cache

src/journal/mmap-cache.c

@@ -72,6 +72,9 @@ struct MMapCache {
         int n_ref;
         unsigned n_windows;
 
+        unsigned n_hit, n_missed;
+
+
         Hashmap *fds;
         Hashmap *contexts;
 
@@ -542,13 +545,19 @@ int mmap_cache_get(
 
         /* Check whether the current context is the right one already */
         r = try_context(m, fd, prot, context, keep_always, offset, size, ret);
-        if (r != 0)
+        if (r != 0) {
+                m->n_hit ++;
                 return r;
+        }
 
         /* Search for a matching mmap */
         r = find_mmap(m, fd, prot, context, keep_always, offset, size, ret);
-        if (r != 0)
+        if (r != 0) {
+                m->n_hit ++;
                 return r;
+        }
+
+        m->n_missed++;
 
         /* Create a new mmap */
         return add_mmap(m, fd, prot, context, keep_always, offset, size, st, ret);
@@ -578,3 +587,15 @@ void mmap_cache_close_context(MMapCache *m, unsigned context) {
 
         context_free(c);
 }
+
+unsigned mmap_cache_get_hit(MMapCache *m) {
+        assert(m);
+
+        return m->n_hit;
+}
+
+unsigned mmap_cache_get_missed(MMapCache *m) {
+        assert(m);
+
+        return m->n_missed;
+}

src/journal/mmap-cache.h

@@ -34,3 +34,6 @@ MMapCache* mmap_cache_unref(MMapCache *m);
 int mmap_cache_get(MMapCache *m, int fd, int prot, unsigned context, bool keep_always, uint64_t offset, size_t size, struct stat *st, void **ret);
 void mmap_cache_close_fd(MMapCache *m, int fd);
 void mmap_cache_close_context(MMapCache *m, unsigned context);
+
+unsigned mmap_cache_get_hit(MMapCache *m);
+unsigned mmap_cache_get_missed(MMapCache *m);

src/journal/sd-journal.c

@@ -1831,8 +1831,10 @@ _public_ void sd_journal_close(sd_journal *j) {
         if (j->inotify_fd >= 0)
                 close_nointr_nofail(j->inotify_fd);
 
-        if (j->mmap)
+        if (j->mmap) {
+                log_debug("mmap cache statistics: %u hit, %u miss", mmap_cache_get_hit(j->mmap), mmap_cache_get_missed(j->mmap));
                 mmap_cache_unref(j->mmap);
+        }
 
         free(j->path);
         free(j->unique_field);