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Thanks for the excellent write up and the time you took to split the review 
into sizable chunks. A full review will take some time. I will start with a 
couple of highlevel questions.

a) Do you have some results of the jmh benachmarks that you might share?

b) If I understand this correctly, you are basically trading one lock with 
another: Instead of waiting on the global storage lock, threads do now have to 
wait on the lock of the `workQueue`. Obviously waiting on that lock is much 
faster as there is less work being done in the critical region protected by 
that lock. However, isn't that a strategy that we could try to use as well? For 
details, see my comment in Part 3.

src/main/java/org/apache/aurora/scheduler/BatchWorker.java (line 58)

    I am wondering if this queue shouldn't have a fixed capacity, so that we 
gain some kind of back pressure throughout the system.

src/main/java/org/apache/aurora/scheduler/BatchWorker.java (line 164)

    `Work` is a subclass of `RepeatableWork` but meant to express something 
non-repeatable. This sounds like a violation of the [Liskov Substitution 
Principle]( https://en.wikipedia.org/wiki/Liskov_substitution_principle). 
    In other words, I feel like `RepeatableWork` should extend `Work` and not 
the other way round.

src/main/java/org/apache/aurora/scheduler/BatchWorker.java (lines 240 - 247)

    A queue could hold vital information that we should not loose during 
scheduler failover (in particular, as we do this once per day).
    Would it be possible to drain the queue completely before exiting here?

src/main/java/org/apache/aurora/scheduler/BatchWorker.java (lines 257 - 259)

    1) Doesn't that effectively result in a busy loop where we are queuing the 
same item over and over until it is can finally be executed? This sounds rather 
    2) This changes the order of work items. Are you sure that no storage 
client requires writes to be in order?

- Stephan Erb

On Sept. 9, 2016, 7:29 p.m., Maxim Khutornenko wrote:
> -----------------------------------------------------------
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> https://reviews.apache.org/r/51759/
> -----------------------------------------------------------
> (Updated Sept. 9, 2016, 7:29 p.m.)
> Review request for Aurora, Joshua Cohen, Stephan Erb, and Zameer Manji.
> 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
  the 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
> -----
>   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/events/PubsubEvent.java 
> 70b5470b9dad1af838b5222cae5ac86487e2f2e4 
>   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 
>   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

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