MB-39745: Scale bgFetchers independently of KVShards

Add a "max_num_bgfetchers" config param for the number of bg fetchers
that we want to create.

Change-Id: Ibaeb5e348239d87d3704a44720b3af02fdc4bf2d
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/136378
Tested-by: Build Bot <[email protected]>
Reviewed-by: Dave Rigby <[email protected]>

Author: BenHuddleston

engines/ep/configuration.json

@@ -664,6 +664,12 @@
             "dynamic": false,
             "type": "size_t"
         },
+        "max_num_bgfetchers": {
+            "default": "0",
+            "descr": "Maximum number of bg fetcher objects (the number of concurrent bg fetch tasks we can run). 0 = auto-configure which means we use the same number as the number of shards (max_num_shards - for historic reasons). See also num_reader_threads.",
+            "dynamic": false,
+            "type": "size_t"
+        },
         "max_num_shards": {
             "default": "0",
             "descr": "Maximum mumber of shards (0 = auto-configure)",

engines/ep/src/ep_bucket.cc

@@ -266,7 +266,16 @@ EPBucket::EPBucket(EventuallyPersistentEngine& theEngine)
             engine.getConfiguration(), stats);
 
     vbMap.enablePersistence(*this);
-    for (size_t i = 0; i < vbMap.getNumShards(); i++) {
+
+    // Pre-7.0.0 BgFetchers were a part of KVShard so keep the same default
+    // scaling.
+    auto configBgFetcherLimit = config.getMaxNumBgfetchers();
+    auto bgFetcherLimit = configBgFetcherLimit == 0 ? vbMap.getNumShards()
+                                                    : configBgFetcherLimit;
+
+    // Limit BgFetchers by the number of vBuckets as any more would be useless.
+    bgFetcherLimit = std::min(bgFetcherLimit, config.getMaxVbuckets());
+    for (size_t i = 0; i < bgFetcherLimit; i++) {
         bgFetchers.emplace_back(std::make_unique<BgFetcher>(*this));
     }
 

engines/ep/tests/ep_testsuite.cc

@@ -7049,6 +7049,7 @@ static enum test_result test_mb19687_fixed(EngineIface* h) {
               "ep_magma_initial_wal_buffer_size",
               "ep_magma_write_cache_ratio",
               "ep_magma_expiry_purger_interval",
+              "ep_max_num_bgfetchers",
               "ep_max_checkpoints",
               "ep_max_failover_entries",
               "ep_max_item_privileged_bytes",
@@ -7264,6 +7265,7 @@ static enum test_result test_mb19687_fixed(EngineIface* h) {
               "ep_items_rm_from_checkpoints",
               "ep_keep_closed_chks",
               "ep_kv_size",
+              "ep_max_num_bgfetchers",
               "ep_max_checkpoints",
               "ep_max_failover_entries",
               "ep_max_item_privileged_bytes",