ring_hash LB: cap ring size to 4096 with channel arg to override (#31692)

* ring_hash LB: cap ring size to 4096 with channel arg to override

* change default max_ring_size value to 4096 in LB policy config

include/grpc/impl/codegen/grpc_types.h

@@ -310,6 +310,10 @@ typedef struct {
   "grpc.service_config_disable_resolution"
 /** LB policy name. */
 #define GRPC_ARG_LB_POLICY_NAME "grpc.lb_policy_name"
+/** Cap for ring size in the ring_hash LB policy.  The min and max ring size
+    values set in the LB policy config will be capped to this value.
+    Default is 4096. */
+#define GRPC_ARG_RING_HASH_LB_RING_SIZE_CAP "grpc.lb.ring_hash.ring_size_cap"
 /** The grpc_socket_mutator instance that set the socket options. A pointer. */
 #define GRPC_ARG_SOCKET_MUTATOR "grpc.socket_mutator"
 /** The grpc_socket_factory instance to create and bind sockets. A pointer. */

src/core/ext/filters/client_channel/lb_policy/ring_hash/ring_hash.cc

@@ -129,6 +129,8 @@ class RingHashLbConfig : public LoadBalancingPolicy::Config {
 // ring_hash LB policy
 //
 
+constexpr size_t kRingSizeCapDefault = 4096;
+
 class RingHash : public LoadBalancingPolicy {
  public:
   explicit RingHash(Args args);
@@ -518,8 +520,12 @@ RingHash::RingHashSubchannelList::RingHashSubchannelList(
   // weights aren't provided, all hosts should get an equal number of hashes. In
   // the case where this number exceeds the max_ring_size, it's scaled back down
   // to fit.
-  const size_t min_ring_size = policy->config_->min_ring_size();
-  const size_t max_ring_size = policy->config_->max_ring_size();
+  const size_t ring_size_cap = args.GetInt(GRPC_ARG_RING_HASH_LB_RING_SIZE_CAP)
+                                   .value_or(kRingSizeCapDefault);
+  const size_t min_ring_size =
+      std::min(policy->config_->min_ring_size(), ring_size_cap);
+  const size_t max_ring_size =
+      std::min(policy->config_->max_ring_size(), ring_size_cap);
   const double scale = std::min(
       std::ceil(min_normalized_weight * min_ring_size) / min_normalized_weight,
       static_cast<double>(max_ring_size));

src/core/ext/filters/client_channel/lb_policy/ring_hash/ring_hash.h

@@ -35,7 +35,7 @@ UniqueTypeName RequestHashAttributeName();
 // hash size validity.
 struct RingHashConfig {
   uint64_t min_ring_size = 1024;
-  uint64_t max_ring_size = 8388608;
+  uint64_t max_ring_size = 4096;
 
   static const JsonLoaderInterface* JsonLoader(const JsonArgs&);
   void JsonPostLoad(const Json& json, const JsonArgs&,