Do Redis lookup in batches in ActiveSession.sessions_from_ids

By doing smaller mget calls to Redis, it can better schedule the
workload. Currently a single mget with a lot of keys can keep Redis busy
for long, while nothing in its queue gets processed.

app/models/active_session.rb

@@ -3,6 +3,8 @@
 class ActiveSession
   include ActiveModel::Model
 
+  SESSION_BATCH_SIZE = 200
+
   attr_accessor :created_at, :updated_at,
     :session_id, :ip_address,
     :browser, :os, :device_name, :device_type,
@@ -106,10 +108,12 @@ class ActiveSession
     Gitlab::Redis::SharedState.with do |redis|
       session_keys = session_ids.map { |session_id| "#{Gitlab::Redis::SharedState::SESSION_NAMESPACE}:#{session_id}" }
 
-      redis.mget(session_keys).compact.map do |raw_session|
-        # rubocop:disable Security/MarshalLoad
-        Marshal.load(raw_session)
-        # rubocop:enable Security/MarshalLoad
+      session_keys.each_slice(SESSION_BATCH_SIZE).flat_map do |session_keys_batch|
+        redis.mget(session_keys_batch).compact.map do |raw_session|
+          # rubocop:disable Security/MarshalLoad
+          Marshal.load(raw_session)
+          # rubocop:enable Security/MarshalLoad
+        end
       end
     end
   end

changelogs/unreleased/64315-mget_sessions_in_chunks.yml

+---
+title: Do Redis lookup in batches in ActiveSession.sessions_from_ids
+merge_request: 30561
+author:
+type: performance

spec/models/active_session_spec.rb

@@ -132,6 +132,19 @@ RSpec.describe ActiveSession, :clean_gitlab_redis_shared_state do
 
       expect(ActiveSession.sessions_from_ids([])).to eq([])
     end
+
+    it 'uses redis lookup in batches' do
+      stub_const('ActiveSession::SESSION_BATCH_SIZE', 1)
+
+      redis = double(:redis)
+      expect(Gitlab::Redis::SharedState).to receive(:with).and_yield(redis)
+
+      sessions = ['session-a', 'session-b']
+      mget_responses = sessions.map { |session| [Marshal.dump(session)]}
+      expect(redis).to receive(:mget).twice.and_return(*mget_responses)
+
+      expect(ActiveSession.sessions_from_ids([1, 2])).to eql(sessions)
+    end
   end
 
   describe '.set' do