From ff22b10e325968fe4a650abb51ac7cd7789c8904 Mon Sep 17 00:00:00 2001
From: Stephan Dollberg <stephan@redpanda.com>
Date: Mon, 16 Dec 2024 10:06:17 +0000
Subject: [PATCH] fetch: Switch to chunked_vector for shard_fetch.requests

`ntp_fetch_config` is 250 bytes. When doing super wide reads of
500-1000k partitions (quite niche) this goes above the oversized alloc
threshold. Switch to chunked_vector.

Switch responses too for uniformity.
---
 src/v/kafka/server/handlers/fetch.cc | 14 +++++++-------
 src/v/kafka/server/handlers/fetch.h  | 18 +++++++++++-------
 2 files changed, 18 insertions(+), 14 deletions(-)

diff --git a/src/v/kafka/server/handlers/fetch.cc b/src/v/kafka/server/handlers/fetch.cc
index f98e318e534bf..2001a35ca34cd 100644
--- a/src/v/kafka/server/handlers/fetch.cc
+++ b/src/v/kafka/server/handlers/fetch.cc
@@ -442,7 +442,7 @@ read_result::memory_units_t reserve_memory_units(
 static void fill_fetch_responses(
   op_context& octx,
   std::vector<read_result> results,
-  const std::vector<op_context::response_placeholder_ptr>& responses,
+  const chunked_vector<op_context::response_placeholder_ptr>& responses,
   op_context::latency_point start_time,
   bool record_latency = true) {
     auto range = boost::irange<size_t>(0, results.size());
@@ -553,7 +553,7 @@ static void fill_fetch_responses(
 static ss::future<std::vector<read_result>> fetch_ntps_in_parallel(
   cluster::partition_manager& cluster_pm,
   const replica_selector& replica_selector,
-  std::vector<ntp_fetch_config> ntp_fetch_configs,
+  chunked_vector<ntp_fetch_config> ntp_fetch_configs,
   read_distribution_probe& read_probe,
   bool foreign_read,
   std::optional<model::timeout_clock::time_point> deadline,
@@ -721,7 +721,7 @@ class fetch_worker {
         std::optional<model::timeout_clock::time_point> deadline;
         // The fetch sub-requests of partitions local to the shard this worker
         // is running on.
-        std::vector<ntp_fetch_config> requests;
+        chunked_vector<ntp_fetch_config> requests;
 
         // References to services local to the shard this worker is running on.
         // They are protected from deletion by the coordinator.
@@ -789,7 +789,7 @@ class fetch_worker {
     };
 
     ss::future<query_results>
-    query_requests(std::vector<ntp_fetch_config> requests) {
+    query_requests(chunked_vector<ntp_fetch_config> requests) {
         // The last visible indexes need to be populated before partitions
         // are read. If they are populated afterwards then the
         // last_visible_index could be updated after the partition is read,
@@ -910,10 +910,10 @@ class fetch_worker {
         size_t total_size{0};
 
         for (;;) {
-            std::vector<ntp_fetch_config> requests;
+            chunked_vector<ntp_fetch_config> requests;
 
             if (first_run) {
-                requests = _ctx.requests;
+                requests = _ctx.requests.copy();
             } else {
                 requests_map.clear();
 
@@ -1191,7 +1191,7 @@ class nonpolling_fetch_plan_executor final : public fetch_plan_executor::impl {
              shard = fetch.shard,
              min_fetch_bytes,
              foreign_read,
-             configs = fetch.requests,
+             configs = fetch.requests.copy(),
              &octx](cluster::partition_manager& mgr) mutable
             -> ss::future<fetch_worker::worker_result> {
                 // Although this and octx are captured by reference across
diff --git a/src/v/kafka/server/handlers/fetch.h b/src/v/kafka/server/handlers/fetch.h
index 5418bb5a2e302..30ba25e69473c 100644
--- a/src/v/kafka/server/handlers/fetch.h
+++ b/src/v/kafka/server/handlers/fetch.h
@@ -10,6 +10,7 @@
  */
 #pragma once
 #include "cluster/rm_stm.h"
+#include "container/fragmented_vector.h"
 #include "container/intrusive_list_helpers.h"
 #include "kafka/protocol/fetch.h"
 #include "kafka/server/handlers/fetch/replica_selector.h"
@@ -329,8 +330,10 @@ struct read_result {
 // struct aggregating fetch requests and corresponding response iterators for
 // the same shard
 struct shard_fetch {
-    explicit shard_fetch(op_context::latency_point start_time)
-      : start_time{start_time} {}
+    explicit shard_fetch(
+      ss::shard_id shard_id, op_context::latency_point start_time)
+      : shard(shard_id)
+      , start_time{start_time} {}
 
     void push_back(
       ntp_fetch_config config, op_context::response_placeholder_ptr r_ph) {
@@ -345,8 +348,8 @@ struct shard_fetch {
     }
 
     ss::shard_id shard;
-    std::vector<ntp_fetch_config> requests;
-    std::vector<op_context::response_placeholder_ptr> responses;
+    chunked_vector<ntp_fetch_config> requests;
+    chunked_vector<op_context::response_placeholder_ptr> responses;
     op_context::latency_point start_time;
 
     friend std::ostream& operator<<(std::ostream& o, const shard_fetch& sf) {
@@ -359,9 +362,10 @@ struct fetch_plan {
     explicit fetch_plan(
       size_t shards,
       op_context::latency_point start_time = op_context::latency_clock::now())
-      : fetches_per_shard(shards, shard_fetch(start_time)) {
-        for (size_t i = 0; i < fetches_per_shard.size(); i++) {
-            fetches_per_shard[i].shard = i;
+      : fetches_per_shard() {
+        fetches_per_shard.reserve(shards);
+        for (size_t i = 0; i < shards; i++) {
+            fetches_per_shard.emplace_back(i, start_time);
         }
     }