gitlab-com/gl-infra/scalability#1217 (closed)
This is part of gitlab-com/gl-infra&463 (closed), where we are working on making the Git HTTP fetches faster at the transport level. PackObjectsHook is one of the RPC's on the critical path. This MR adds a new RPC PackObjectsHookWithSidechannel that uses a Unix socket sidechannel for the data transfer. This avoids the overhead of gRPC.
On its own, this will reduce CPU usage on Gitaly servers that serve a lot of fetch traffic, but we will not see the full benefits until we have improved PostUploadPack too. That is the other half of gitlab-com/gl-infra&463 (closed).