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Admission Control in Impala

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  • 1. 1 Admission Control in Impala Matthew Jacobs | @mattjacobs | mj@cloudera.com
  • 2. 2 ©2014 Cloudera, Inc. All rights reserved. • Too many concurrent queries -> oversubscription • All queries take more time • Application layer can throttle queries? • Not much you can do before Impala 1.3 What’s the Problem
  • 3. 3 ©2014 Cloudera, Inc. All rights reserved. • Add an admission control mechanism to Impala! • Throttle incoming requests • Queues requests when workload increases • Queued requests executed when resources available So what do we do?
  • 4. 4 ©2014 Cloudera, Inc. All rights reserved. • Yarn is a resource manager for Hadoop • Assumes jobs are composed of tasks, tasks can be restarted • Need to ask for all resources up front, resources “trickle in” • Non-trivial overhead: job creates “application master” (AM) • But cost is small compared to long batch jobs • Great for MR, things like MR • Not good for • Low-latency, high volume workloads • Gang scheduling, “parts of jobs” can’t be restarted What about Yarn?
  • 5. 5 ©2014 Cloudera, Inc. All rights reserved. • “Long Lived Application Master” • Long running AMs • Create fake requests to acquire necessary resources • Provides a “gang scheduling” abstraction, waits for all resources • Offers a resource expansion mechanism -> don’t need to ask for all up front • Offers a throttling mechanism • Caches Yarn containers -> lower latency • Looks like a square peg in a round hole… • To be fair, multi-level scheduling is a hard problem! Llama Bridges the Gap
  • 6. 6 ©2014 Cloudera, Inc. All rights reserved. • Good for Impala sharing resources with other frameworks • Good general purpose resource mgmt solution However: • Not everyone wants/needs to run Yarn and Llama • Still requires round-trips to a central server • Increases query latency • Unlikely to scale for highest latency/throughput requirements • Impala should have a fast, built in throttling mechanism Impala + Llama + Yarn?
  • 7. 7 ©2014 Cloudera, Inc. All rights reserved. • Throttle number of concurrent requests or memory • Fast • Decentralized • Works without Yarn/Llama • Works with CDH4/CDH5 Impala Admission Control
  • 8. 8 ©2014 Cloudera, Inc. All rights reserved. • Configure one or more resource “pools” • Max # concurrent queries, max memory, max queue size • Each Impalad capable of making admission decisions • No new single bottleneck/single point of failure • Incoming queries are executed, queued, or rejected • Queue if too many queries OR not enough memory • Reject if queue is full Design Overview
  • 9. 9 ©2014 Cloudera, Inc. All rights reserved. • Requests admitted or queued locally • Each Impalad keeps track of local state • # queries, pool memory, local queue size • Disseminates local stats via statestore -> global state • Uses cached global state in admission decisions • Decisions are fast; negligible impact on query latency • No single point of failure Localized Admission Decisions
  • 10. 10 ©2014 Cloudera, Inc. All rights reserved. • Using cached global state -> may “over-admit” • E.g. multiple impalads think 1 request can be admitted and admit before receiving updated state • Configured pool limits are “soft” limits • Fn(Submission rate, distribution across impalads) • Not a big problem in practice • May occasionally admit a few extra queries • Can increase statestore heartbeat frequency • Can add some buffer to configured pool limits Localized Admission Decisions (II)
  • 11. 11 ©2014 Cloudera, Inc. All rights reserved. • Max memory • Many workloads are limited by memory • Impalads kill queries when running out of memory, anyway • Max number of concurrent queries • Generic mechanism, not resource specific (e.g. memory) • Not as good if workload is heterogeneous • Queries may still be killed if impalads run out of memory Pool Limits
  • 12. 12 Memory Limits • Impalads track memory hierarchically • Per-process memory • Queries killed when limit is hit • Per-pool memory • For admission control • Per-query memory Process Pool1 Query1 Query2 Pool2 ©2014 Cloudera, Inc. All rights reserved.
  • 13. 13 ©2014 Cloudera, Inc. All rights reserved. • Admission decisions need more than memory usage • Incoming queries use no memory yet • Queries recently admitted haven’t ramped up yet • Use memory estimates from planning • Estimate pool memory usage with actual usage & estimates • Accounts for future memory usage of recently started queries Admit if: Pool mem estimate + query mem estimate < pool limit Memory Limits (II)
  • 14. 14 ©2014 Cloudera, Inc. All rights reserved. • Not perfect, query mem estimates are wrong • Hard problem; never have perfect estimates from planning • Usually overly conservative • Leads to underutilization • But at least queries won’t be killed • Less likely to hit process mem limit • Workarounds • Increase pool mem limit • Override with “MEM_LIMIT” query option • Future improvement: Update estimates as query executes • Query mem usage will approach the updated estimate Memory Limits (III)
  • 15. 15 ©2014 Cloudera, Inc. All rights reserved. • Modeled after Yarn resource queues • Same configuration as Yarn queues • Have a single configuration for Yarn & Impala • Usually want to have the same resource allocations mapped to an organization • E.g. HR gets 10%, Finance gets 30%, Eng gets 60% Request Pools
  • 16. 16 ©2014 Cloudera, Inc. All rights reserved. • Users are mapped to pools using the placement policy • Users are authorized using the specified ACLs • Pools are defined hierarchically • ACLs are inherited • Currently only enforces limits on leaf pools (IMPALA-905) Request Pools (II)
  • 17. 17 ©2014 Cloudera, Inc. All rights reserved. • Uses Yarn + Llama configs • Yarn fair scheduler allocation configuration (fair- scheduler.xml) • Llama configuration (llama-site.xml) • Only some of the configuration properties are used • See the documentation for sample config files • Cloudera Manager has a nice UI to configure • No need to touch the xml files Request Pool General Configuration
  • 18. 18 ©2014 Cloudera, Inc. All rights reserved.
  • 19. 19 ©2014 Cloudera, Inc. All rights reserved. Placement Rule Configuration Please change the default values
  • 20. 20 ©2014 Cloudera, Inc. All rights reserved. • If only 1 pool is needed, a separate (easy) configuration path exists • Uses a single “default” pool • No Yarn/Llama configs involved (not even accepted) • Configure the pool limits with impalad flags: • default_pool_max_queued • default_pool_max_requests • default_pool_mem_limit • Doesn’t work with CM5.0, fixed in CM5.0.1 Easy Config Path (Singleton Pool Only)
  • 21. 21 ©2014 Cloudera, Inc. All rights reserved. Submitting to a Pool
  • 22. 22 ©2014 Cloudera, Inc. All rights reserved. • Rejections and timeouts return error messages • Metrics • Exposed in impalad web UI: /metrics • Will be available in CM5.1 • Query profile has admission result • Impalad logs have lots of useful information “Debugging” Admission Control Decisions admission-controller.cc:259] Schedule for id=c541aae43af74ed1:afdec812127f8097 in pool_name=root.test/admin PoolConfig(max_requests=20 max_queued=50 mem_limit=-1.00 B) query cluster_mem_estimate=42.00 MB admission-controller.cc:265] Stats: pool=root.test/admin Total(num_running=20, num_queued=7, mem_usage=239.07 MB, mem_estimate=800.00 MB) Local(num_running=20, num_queued=7, mem_usage=239.07 MB, mem_estimate=800.00 MB) admission-controller.cc:303] Queuing, query id=c541aae43af74ed1:afdec812127f8097
  • 23. 23 ©2014 Cloudera, Inc. All rights reserved. Metrics
  • 24. 24 ©2014 Cloudera, Inc. All rights reserved. Query Profile Information
  • 25. 25 ©2014 Cloudera, Inc. All rights reserved. • Queue timeout • Defaults to 60sec, change with --queue_wait_timeout_ms • Running with Yarn/Llama • Same configs: “hard limits” enforced by Yarn+Llama • Disabled by default for CDH4 • Hue (<CDH4.6) doesn’t close queries • Enable with impalad flag (see --disable_admission_control) Some Notes
  • 26. 26 ©2014 Cloudera, Inc. All rights reserved. Matthew Jacobs @mattjacobs mj@cloudera.com

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