Speedup INFO by counting client memory incrementally.faster-info

Related to #5145.

Design note: clients may change type when they turn into replicas or are
moved into the Pub/Sub category and so forth. Moreover the recomputation
of the bytes used is problematic for obvious reasons: it changes
continuously, so as a conservative way to avoid accumulating errors,
each client remembers the contribution it gave to the sum, and removes
it when it is freed or before updating it with the new memory usage.

1 files changed, 25 insertions, 0 deletions

diff --git a/src/server.c b/src/server.c
index 56feb09a3..996e0f5d2 100644
--- a/src/server.c
+++ b/src/server.c
@@ -1593,6 +1593,28 @@ int clientsCronTrackExpansiveClients(client *c) {
     return 0; /* This function never terminates the client. */
 }
 
+/* Iterating all the clients in getMemoryOverheadData() is too slow and
+ * in turn would make the INFO command too slow. So we perform this
+ * computation incrementally and track the (not instantaneous but updated
+ * to the second) total memory used by clients using clinetsCron() in
+ * a more incremental way (depending on server.hz). */
+int clientsCronTrackClientsMemUsage(client *c) {
+    size_t mem = 0;
+    int type = getClientType(c);
+    mem += getClientOutputBufferMemoryUsage(c);
+    mem += sdsAllocSize(c->querybuf);
+    mem += sizeof(client);
+    /* Now that we have the memory used by the client, remove the old
+     * value from the old categoty, and add it back. */
+    server.stat_clients_type_memory[c->client_cron_last_memory_type] -=
+        c->client_cron_last_memory_usage;
+    server.stat_clients_type_memory[type] += mem;
+    /* Remember what we added and where, to remove it next time. */
+    c->client_cron_last_memory_usage = mem;
+    c->client_cron_last_memory_type = type;
+    return 0;
+}
+
 /* Return the max samples in the memory usage of clients tracked by
  * the function clientsCronTrackExpansiveClients(). */
 void getExpansiveClientsInfo(size_t *in_usage, size_t *out_usage) {
@@ -1653,6 +1675,7 @@ void clientsCron(void) {
         if (clientsCronHandleTimeout(c,now)) continue;
         if (clientsCronResizeQueryBuffer(c)) continue;
         if (clientsCronTrackExpansiveClients(c)) continue;
+        if (clientsCronTrackClientsMemUsage(c)) continue;
     }
 }
 
@@ -2792,6 +2815,8 @@ void initServer(void) {
     server.stat_rdb_cow_bytes = 0;
     server.stat_aof_cow_bytes = 0;
     server.stat_module_cow_bytes = 0;
+    for (int j = 0; j < CLIENT_TYPE_COUNT; j++)
+        server.stat_clients_type_memory[j] = 0;
     server.cron_malloc_stats.zmalloc_used = 0;
     server.cron_malloc_stats.process_rss = 0;
     server.cron_malloc_stats.allocator_allocated = 0;