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(Updated Sept. 14, 2016, 4:47 p.m.) Review request for Aurora, Joshua Cohen, Stephan Erb, and Zameer Manji. Changes ------- Javadoc changes and minor refactoring. Repository: aurora Description ------- This is the first (out of 3) patches intending to reduce storage write lock contention and as such improve overall system write throughput. It introduces the `BatchWorker` and migrates the majority of storage writes due to task status change events to use `TaskEventBatchWorker`. #####Problem Our current storage system writes effectively behave as `SERIALIZABLE` transaction isolation level in SQL terms. This means all writes require exclusive access to the storage and no two transactions can happen in parallel [1]. While it certainly simplifies our implementation, it creates a single hotspot where multiple threads are competing for the storage write access. This type of contention only worsens as the cluster size grows, more tasks are scheduled, more status updates are processed, more subscribers are listening to status updates and etc. Eventually, the scheduler throughput (and especially task scheduling) becomes degraded to the extent that certain operations wait much longer (4x and more) for the lock acquisition than it takes to process their payload when inside the transaction. Some ops (like event processing) are generally tolerant of these types of delays. Others - not as much. The task scheduling suffers the most as backing up the scheduling queue directly affects t he Median Time To Assigned (MTTA). #####Remediation Given the above, it's natural to assume that reducing the number of write transactions should help reducing the lock contention. This patch introduces a generic `BatchWorker` service that delivers a "best effort" batching approach by redirecting multiple individual write requests into a single FIFO queue served non-stop by a single dedicated thread. Every batch shares a single write transaction thus reducing the number of potential write lock requests. To minimize wait-in-queue time, items are dispatched immediately and the max number of items is bounded. There are a few `BatchWorker` instances specialized on particular workload types: task even processing, cron scheduling and task scheduling. Every instance can be tuned independently (max batch size) and provides specialized metrics helping to monitor each workload type perf. #####Results The proposed approach has been heavily tested in production and delivered the best results. The lock contention latencies got down between 2x and 5x depending on the cluster load. A number of other approaches tried but discarded as not performing well or even performing much worse than the current master: - Clock-driven batch execution - every batch is dispatched on a time schedule - Max batch with a deadline - a batch is dispatched when max size is reached OR a timeout expires - Various combinations of the above - some `BatchWorkers` are using clock-driven execution while others are using max batch with a deadline - Completely non-blocking (event-based) completion notification - all call sites are notified of item completion via a `BatchWorkCompleted` event Happy to provide more details on the above if interested. #####Upcoming The introduction of the `BatchWorker` by itself was not enough to substantially improve the MTTA. It, however, paves the way for the next phase of scheduling perf improvement - taking more than 1 task from a given `TaskGroup` in a single scheduling round (coming soon). That improvement wouldn't deliver without decreasing the lock contention first. Note: it wasn't easy to have a clean diff split, so some functionality in `BatchWorker` (e.g.: `executeWithReplay`) appears to be unused in the current patch but will become obvious in the part 2 (coming out shortly). [1] - https://github.com/apache/aurora/blob/f6ac13b169aaad5aad73ef3cc72873781e30a705/src/main/java/org/apache/aurora/scheduler/storage/log/LogStorage.java#L540-L555 Diffs (updated) ----- src/main/java/org/apache/aurora/scheduler/BatchWorker.java PRE-CREATION src/main/java/org/apache/aurora/scheduler/SchedulerModule.java 4a7ef0b1b90607f68d89fe8e207f42c42a8c56a0 src/main/java/org/apache/aurora/scheduler/pruning/TaskHistoryPruner.java f07746c2b990c1c2235e99f9e4775fc84f9c27b1 src/main/java/org/apache/aurora/scheduler/scheduling/TaskThrottler.java bbd971a2aa8a96cf79edd879ad60e1bebd933d79 src/main/java/org/apache/aurora/scheduler/state/MaintenanceController.java 3c7cda09ab292d696070ca4d9dfedb1f6f71b0fe src/main/java/org/apache/aurora/scheduler/updater/JobUpdateControllerImpl.java 594bb6219294dcc77d48dcad14e2a6f9caa0c534 src/test/java/org/apache/aurora/scheduler/BatchWorkerTest.java PRE-CREATION src/test/java/org/apache/aurora/scheduler/pruning/TaskHistoryPrunerTest.java 99c27e8012f10a67ce5f1b84d258e7a5608995c7 src/test/java/org/apache/aurora/scheduler/scheduling/TaskThrottlerTest.java 7d104aa2ea4a4d99be4711f666d18beca238284e src/test/java/org/apache/aurora/scheduler/state/MaintenanceControllerImplTest.java 94f5ca565476f62d72879837a0e7dafabcf30432 src/test/java/org/apache/aurora/scheduler/testing/BatchWorkerUtil.java PRE-CREATION src/test/java/org/apache/aurora/scheduler/updater/JobUpdaterIT.java 196df4754b553f05e50b66ad2f84271901bc9eba Diff: https://reviews.apache.org/r/51759/diff/ Testing ------- All types of testing including deploying to test and production clusters. Thanks, Maxim Khutornenko
