Instrument usage stack v9 with read stream info#42
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Introduce TriggerInstrumentation to capture trigger timing and firings (previously counted in "ntuples"), to aid a future refactoring that splits out all Instrumentation fields beyond timing and WAL/buffers into more specific structs. Author: Lukas Fittl <lukas@fittl.com> Reviewed-by: Discussion:
Previously different places (e.g. query "total time") were repurposing the Instrumentation struct initially introduced for capturing per-node statistics during execution. This overuse of the same struct is confusing, e.g. by cluttering calls of InstrStartNode/InstrStopNode in unrelated code paths, and prevents future refactorings. Instead, simplify the Instrumentation struct to only track time and WAL/buffer usage. Similarly, drop the use of InstrEndLoop outside of per-node instrumentation - these calls were added without any apparent benefit since the relevant fields were never read. Introduce the NodeInstrumentation struct to carry forward the per-node instrumentation information. WorkerInstrumentation is renamed to WorkerNodeInstrumentation for clarity. In passing, drop the "n" argument to InstrAlloc, as all remaining callers need exactly one Instrumentation struct. Author: Lukas Fittl <lukas@fittl.com> Reviewed-by: Discussion:
…ith INSTR_* macros This encapsulates the ownership of these globals better, and will allow a subsequent refactoring. Author: Lukas Fittl <lukas@fittl.com> Reviewed-by: Andres Freund <andres@anarazel.de> Discussion: https://www.postgresql.org/message-id/flat/CAP53PkzZ3UotnRrrnXWAv%3DF4avRq9MQ8zU%2BbxoN9tpovEu6fGQ%40mail.gmail.com#fc7140e8af21e07a90a09d7e76b300c4
This adds regression tests that cover some of the expected behaviour around the buffer statistics reported in EXPLAIN ANALYZE, specifically how they behave in parallel query, nested function calls and abort situations. Testing this is challenging because there can be different sources of buffer activity, so we rely on temporary tables where we can to prove that activity was captured and not lost. This supports a future commit that will rework some of the instrumentation logic that could cause areas covered by these tests to fail. Author: Lukas Fittl <lukas@fittl.com> Reviewed-by: Discussion:
Previously, in order to determine the buffer/WAL usage of a given code section, we utilized continuously incrementing global counters that get updated when the actual activity (e.g. shared block read) occurred, and then calculated a diff when the code section ended. This resulted in a bottleneck for executor node instrumentation specifically, with the function BufferUsageAccumDiff showing up in profiles and in some cases adding up to 10% overhead to an EXPLAIN (ANALYZE, BUFFERS) run. Instead, introduce a stack-based mechanism, where the actual activity writes into the current stack entry. In the case of executor nodes, this means that each node gets its own stack entry that is pushed at InstrStartNode, and popped at InstrEndNode. Stack entries are zero initialized (avoiding the diff mechanism) and get added to their parent entry when they are finalized, i.e. no more modifications can occur. To correctly handle abort situations, any use of instrumentation stacks must involve either a top-level QueryInstrumentation struct, and its associated InstrQueryStart/InstrQueryStop helpers (which use resource owners to handle aborts), or the Instrumentation struct itself with dedicated PG_TRY/PG_FINALLY calls that ensure the stack is in a consistent state after an abort. This also drops the global pgBufferUsage, any callers interested in measuring buffer activity should instead utilize InstrStart/InstrStop. The related global pgWalUsage is kept for now due to its use in pgstat to track aggregate WAL activity and heap_page_prune_and_freeze for measuring FPIs. Author: Lukas Fittl <lukas@fittl.com> Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com> Discussion: https://www.postgresql.org/message-id/flat/CAP53PkxrmpECzVFpeeEEHDGe6u625s%2BYkmVv5-gw3L_NDSfbiA%40mail.gmail.com#cb583a08e8e096aa1f093bb178906173
For most queries, the bulk of the overhead of EXPLAIN ANALYZE happens in ExecProcNodeInstr when starting/stopping instrumentation for that node. Previously each ExecProcNodeInstr would check which instrumentation options are active in the InstrStartNode/InstrStopNode calls, and do the corresponding work (timers, instrumentation stack, etc.). These conditionals being checked for each tuple being emitted add up, and cause non-optimal set of instructions to be generated by the compiler. Because we already have an existing mechanism to specify a function pointer when instrumentation is enabled, we can instead create specialized functions that are tailored to the instrumentation options enabled, and avoid conditionals on subsequent ExecProcNodeInstr calls. This results in the overhead for EXPLAIN (ANALYZE, TIMING OFF, BUFFERS OFF) for a stress test with a large COUNT(*) that does many ExecProcNode calls from ~ 20% on top of actual runtime to ~ 3%. When using BUFFERS ON the same query goes from ~ 20% to ~ 10% on top of actual runtime. Author: Lukas Fittl <lukas@fittl.com> Reviewed-by: Discussion:
This sets up a separate instrumentation stack that is used whilst an Index Scan does scanning on the table, for example due to additional data being needed. EXPLAIN ANALYZE will now show "Table Buffers" that represent such activity. The activity is also included in regular "Buffers" together with index activity and that of any child nodes. Author: Lukas Fittl <lukas@fittl.com> Suggested-by: Andres Freund <andres@anarazel.de> Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com> Discussion: https://www.postgresql.org/message-id/flat/CAP53PkxrmpECzVFpeeEEHDGe6u625s%2BYkmVv5-gw3L_NDSfbiA%40mail.gmail.com#cb583a08e8e096aa1f093bb178906173
This is intended for testing instrumentation related logic as it pertains to the top level stack that is maintained as a running total. There is currently no in-core user that utilizes the top-level values in this manner, and especially during abort situations this helps ensure we don't lose activity due to incorrect handling of unfinalized node stacks.
In a subsequent commit read_stream.c will use this as an input to the read ahead distance. Author: Reviewed-by: Discussion: https://postgr.es/m/ Backpatch:
This simplifies the allocations a bit, since we don't need to separately allocate WAL and buffer usage, and allows the easier addition of a planned third struct in Instrumentation. Author: Lukas Fittl <lukas@fittl.com> Reviewed-by: Discussion: https://postgr.es/m/
This adds details about AIO / prefetch for executor nodes using the ReadStream API, currently SeqScan and BitmapHeapScan, into the new IOUsage information added to Instrumentation. This can be viewed through the new EXPLAIN (IO) information, or could be tracked by other interested callers through the stack-based instrumentation mechanism. The ReadStream tracks the statistics unconditionally, i.e. even outside EXPLAIN ANALYZE etc. The amount of statistics is trivial (a handful of integer counters), it's not worth gating this by a flag. Author: Lukas Fittl <lukas@fittl.com> Author: Tomas Vondra <tomas@vondra.me> Reviewed By: Discussion: https://www.postgresql.org/message-id/flat/a177a6dd-240b-455a-8f25-aca0b1c08c6e%40vondra.me
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