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kvflowcontrol,raftlog: interfaces for replication control
Follower replication work, today, is not subject to admission control. It consumes IO tokens without waiting, which both (i) does not prevent the LSM from being inverted, and (ii) can cause priority inversion where low-pri follower write work ends up causing IO token exhaustion, which in turn causes throughput and latency impact for high-pri non-follower write work on that same store. This latter behavior was especially noticeble with large index backfills (cockroachdb#82556) where >2/3rds of write traffic on stores could be follower work for large AddSSTs, causing IO token exhaustion for regular write work being proposed on those stores. We last looked at this problem as part of cockroachdb#79215, settling on cockroachdb#83851 which pauses replication traffic to stores close to exceeding their IO overload threshold (data that's periodically gossiped). In large index backfill experiments we found this to help slightly, but it's still a coarse and imperfect solution -- we're deliberately causing under-replication instead of being able to shape the rate of incoming writes for low-pri work closer to the origin. As part of cockroachdb#95563 we're introducing machinery for "replication admission control" -- end-to-end flow control for replication traffic. With it we expect to no longer need to bypass follower write work in admission control and solve the issues mentioned above. Some small degree of familiarity with the design is assumed below. In this proto{col,buf}/interface-only commit and the previous raft log encoding commit, we introduce: 1. Package kvflowcontrol{,pb}, which will provide flow control for replication traffic in KV. It will be part of the integration layer between KV and admission control. In it we have a few central interfaces: - kvflowcontrol.Controller, held at the node-level and holds all kvflowcontrol.Tokens for each kvflowcontrol.Stream (one per store we're sending raft traffic to and tenant we're sending it for). - kvflowcontrol.Handle, which will held at the replica-level (only on those who are both leaseholder and raft leader), and will be used to interface with the node-level kvflowcontrol.Controller. When replicating log entries, these replicas choose the log position (term+index) the data is to end up at, and use this handle to track the token deductions on a per log position basis. Later when freeing up tokens (after being informed of said log entries being admitted on the receiving end of the stream), it's done so by specifying the log position up to which we free up all deducted tokens. type Controller interface { Admit(admissionpb.WorkPriority, ...Stream) DeductTokens(admissionpb.WorkPriority, Tokens, ...Stream) ReturnTokens(admissionpb.WorkPriority, Tokens, ...Stream) } type Handle interface { Admit(admissionpb.WorkPriority) DeductTokensFor(admissionpb.WorkPriority, kvflowcontrolpb.RaftLogPosition, Tokens) ReturnTokensUpto(admissionpb.WorkPriority, kvflowcontrolpb.RaftLogPosition, Stream) TrackLowWater(kvflowcontrolpb.RaftLogPosition, Stream) Close() } 2. kvflowcontrolpb.RaftAdmissionMeta and relevant encoding/decoding routines. RaftAdmissionMeta is 'embedded' within a kvserverpb.RaftCommand, and includes necessary AC metadata on a per raft entry basis. Entries that contain this metadata will make use of the AC-specific raft log entry encodings described earlier. The AC metadata is decoded below-raft when looking to admit the write work. Also included is the node where this command originated, who wants to eventually learn of this command's admission. message RaftAdmissionMeta { int32 admission_priority = ...; int64 admission_create_time = ...; int32 admission_origin_node = ...; } 3. kvflowcontrolpb.AdmittedRaftLogEntries, which now features in kvserverpb.RaftMessageRequest, the unit of what's sent back-and-forth between two nodes over their two uni-directional raft transport streams. AdmittedRaftLogEntries, just like raft heartbeats, is coalesced information about all raft log entries that were admitted below raft. We'll use the origin node encoded in raft entry (admission_origin_node from from above) to know where to send these to, which in turn will release flow tokens that were acquired when replicating the original log entries. message AdmittedRaftLogEntries { int64 range_id = ...; int32 admission_priority = ...; RaftLogPosition up_to_raft_log_position = ...; uint64 store_id = ...; } message RaftLogPosition { uint64 term = ...; uint64 index = ...; } 4. kvflowcontrol.Dispatch, which is used to dispatch information about admitted raft log entries (see AdmittedRaftLogEntries from above) to specific nodes where (i) said entries originated, (ii) flow tokens were deducted and (iii) are waiting to be returned. The interface is also used to read pending dispatches, which will be used in the raft transport layer when looking to piggyback information on traffic already bound to specific nodes. Since timely dispatching (read: piggybacking) is not guaranteed, we allow querying for all long-overdue dispatches. The interface looks roughly like: type Dispatch interface { Dispatch(roachpb.NodeID, kvflowcontrolpb.AdmittedRaftLogEntries) PendingDispatch() []roachpb.NodeID PendingDispatchFor(roachpb.NodeID) []kvflowcontrolpb.AdmittedRaftLogEntries } 5. Two new encodings for raft log entries, EntryEncoding{Standard,Sideloaded}WithAC. Raft log entries have prefix byte that informs decoding routines how to interpret the subsequent bytes. To date we've had two, EntryEncoding{Standard,Sideloaded} (now renamed to EntryEncoding{Standard,Sideloaded}WithoutAC), to indicate whether the entry came with sideloaded data (these are typically AddSSTs, the storage for which is treated differently for performance). Our two additions here will be used to indicate whether the particular entry is subject to replication admission control. If so, right as we persist entries into the raft log storage, we'll admit the work without blocking. - We'll come back to this non-blocking admission in the AdmitRaftEntry section below, even though the implementation is left for a future PR. - The decision to use replication admission control happens above raft, and AC-specific metadata is plumbed down as part of the marshaled raft command, as described for RaftAdmissionMeta above. 6. An unused version gate (V23_1UseEncodingWithBelowRaftAdmissionData) to use replication admission control. Since we're using a different prefix byte for raft commands (see EntryEncodings above), one not recognized in earlier CRDB versions, we need explicit versioning. 7. AdmitRaftEntry, on the kvadmission.Controller interface. We'll use this as the integration point for log entries received below raft, right as they're being written to storage. This will be non-blocking since we'll be below raft in the raft.Ready() loop, and will effectively enqueue a "virtual" work item in underlying StoreWorkQueue mediating store IO. This virtual work item is what later gets dequeued once the store granter informs the work queue of newly available IO tokens. For standard work queue ordering, our work item needs to include the create time and admission pri. The tenant ID is plumbed to find the right tenant heap to queue it under (for inter-tenant isolation); the store ID to find the right store work queue on multi-store nodes. The raftpb.Entry encodes within it its origin node (see RaftAdmissionMeta above), which is used post-admission to inform the right node of said admission. It looks like: // AdmitRaftEntry informs admission control of a raft log entry being // written to storage. AdmitRaftEntry(roachpb.TenantID, roachpb.StoreID, roachpb.RangeID, raftpb.Entry) Release note: None
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