Try to trim strings only when applicable (#11817)

As `sdsRemoveFreeSpace` have an impact on performance even if it is a no-op (see details at #11508). 
Only call the function when there is a possibility that the string contains free space.
* For strings coming from the network, it's only if they're bigger than PROTO_MBULK_BIG_ARG
* For strings coming from scripts, it's only if they're smaller than LUA_CMD_OBJCACHE_MAX_LEN
* For strings coming from modules, it could be anything.

Co-authored-by: Oran Agra <oran@redislabs.com>
Co-authored-by: sundb <sundbcn@gmail.com>

1 files changed, 16 insertions, 17 deletions

diff --git a/src/object.c b/src/object.c
index f64695061..c89f5aa85 100644
--- a/src/object.c
+++ b/src/object.c
@@ -609,14 +609,20 @@ int isObjectRepresentableAsLongLong(robj *o, long long *llval) {
 }
 
 /* Optimize the SDS string inside the string object to require little space,
- * in case there is more than 10% of free space at the end of the SDS
- * string. This happens because SDS strings tend to overallocate to avoid
- * wasting too much time in allocations when appending to the string. */
-void trimStringObjectIfNeeded(robj *o) {
-    if (o->encoding == OBJ_ENCODING_RAW &&
-        sdsavail(o->ptr) > sdslen(o->ptr)/10)
-    {
-        o->ptr = sdsRemoveFreeSpace(o->ptr, 0);
+ * in case there is more than 10% of free space at the end of the SDS. */
+void trimStringObjectIfNeeded(robj *o, int trim_small_values) {
+    if (o->encoding != OBJ_ENCODING_RAW) return;
+    /* A string may have free space in the following cases:
+     * 1. When an arg len is greater than PROTO_MBULK_BIG_ARG the query buffer may be used directly as the SDS string.
+     * 2. When utilizing the argument caching mechanism in Lua. 
+     * 3. When calling from RM_TrimStringAllocation (trim_small_values is true). */
+    size_t len = sdslen(o->ptr);
+    if (len >= PROTO_MBULK_BIG_ARG ||
+        trim_small_values||
+        (server.executing_client && server.executing_client->flags & CLIENT_SCRIPT && len < LUA_CMD_OBJCACHE_MAX_LEN)) {
+        if (sdsavail(o->ptr) > len/10) {
+            o->ptr = sdsRemoveFreeSpace(o->ptr, 0);
+        }
     }
 }
 
@@ -685,15 +691,8 @@ robj *tryObjectEncoding(robj *o) {
     }
 
     /* We can't encode the object...
-     *
-     * Do the last try, and at least optimize the SDS string inside
-     * the string object to require little space, in case there
-     * is more than 10% of free space at the end of the SDS string.
-     *
-     * We do that only for relatively large strings as this branch
-     * is only entered if the length of the string is greater than
-     * OBJ_ENCODING_EMBSTR_SIZE_LIMIT. */
-    trimStringObjectIfNeeded(o);
+     * Do the last try, and at least optimize the SDS string inside */
+    trimStringObjectIfNeeded(o, 0);
 
     /* Return the original object. */
     return o;