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Concurrency basics For Elixir-based Systems
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So, what’s concurrency?
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Sequential Execution (3 functions, 1 thread)
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Sequential Execution (3 functions, 1 thread) Concurrent Execution (3 functions, 3 threads)
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Sequential Execution (3 functions, 1 thread) Concurrent Execution (3 functions, 3 threads) Preemptive scheduling
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Where’s the benefit?
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Req1 Req2 Req3 Resp Sequential Execution time Waiting time
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Req1 Req2 Req3 Resp Req1 Resp Req2 Req3 Sequential Concurrent Execution time Waiting time
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CPU bound Re Re Re Res Re Res Re Re I/O bound
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Concurrent or Parallel What’s the difference?
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Concurrent Execution (3 functions, 3 threads)
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Concurrent Execution (3 functions, 3 threads) Parallel Execution (3 functions, 3 threads, 2 cores) core 1 core 2
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root@kingschat-api-c8f8d6b76-4j65j:/app# nproc 12 root@tahmeel-api-prod-b5979bdc6-q5wz6:/# nproc 1 How many cores?
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Concurrent Execution (3 functions, 3 threads) Parallel Execution (3 functions, 3 threads, 2 cores) core 1 core 2 (by default) One erlang scheduler per core
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:observer_cli.start()
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Req1 Req2 Req3 Resp Req1 Resp Req2 Req3 Sequential Concurrent Execution time Waiting time Req1 Resp Req2 Req3 Parallel
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Sequential execution
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Phoenix Request Req 1
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Phoenix Request Resp
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Phoenix Request Req 2
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Phoenix Request Resp
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Phoenix Request Req 3
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Phoenix Request Resp
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Concurrent execution
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Phoenix Request
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Phoenix Request Task 1 Task 2 Task 3
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Phoenix Request Task 1 Task 2 Task 3 Req 1 Req 2 Req 3
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Phoenix Request Task 1 Task 2 Task 3 Resp Resp Resp
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Phoenix Request Task 1 Task 2 Task 3
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R1 APP Server DB Server (3 cores) R2 R1 R2 Time Execution time Waiting time
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R1 APP Server DB Server (3 cores) Send resp R2 R3 R1 R2 R3 Time Execution time Waiting time
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How much can we gain?
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Amdahl’s Law
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Amdahl’s Law
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Amdahl’s Law in a nutshell The more synchronisation, the less benefit from multiple cores
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R1 APP Server Send resp R2 R3 R1 R2 R3 Time Execution time Waiting time Almost 100% parallel (almost no synchronisation) DB Server (3 cores)
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But…
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R1 APP Server Send resp R2 R3 R1 R2 R3 Time Execution time Waiting time This is not constant DB Server (3 cores)
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R1 APP Server Send resp R2 R3 R1 R2 R3 Time Execution time Waiting time This is not infinite DB Server (3 cores)
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R1 APP Server R2 R3 R1 R2 R3 R4 Time Execution time Waiting time DB Server (3 cores)
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R1 APP Server R2 R3 R1 R2 R3 R4 Time Execution time Waiting time DB Server (3 cores)
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R1 APP Server R2 R3 R1 R2 R3 R4 R4 Time Execution time Waiting time DB Server (3 cores)
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R1 APP Server R2 R3 R1 R2 R3 R4 R4 Time Execution time Waiting time DB Server (3 cores)
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R1 APP Server R2 R3 R1 R2 R3 R4 R4 Time Execution time Waiting time DB Server (3 cores)
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R1 APP Server Send resp R2 R3 R1 R2 R3 R4 R4 Time Execution time Waiting time DB Server (3 cores)
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R1 APP Server R2 R3 R1 R2 R3 R4 R4 Time Execution time Waiting time R5 R6 R7 R5 R6 R7 DB Server (3 cores)
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Phoenix Request Task 1 Task 2 Task 3 Req 1 Req 2 Req 3 Remember this?
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This isn’t exactly true
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Connection pool (Prevents from overworking the DB)
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Pool Manager (Blocks until a free worker is available)
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Pool Manager (Blocks until a free worker is available)
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It gets worse
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Pool Manager Mailbox Has to be synchronised
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Pool Manager Message Passing Is just copying data in shared memory
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Pool Manager Remember semaphores?
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Logger Metrics Sentry
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Network stack
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Network stack
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Network stack
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Network stack Sentry Metrics
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OS Threads (Garbage Collection) Data Bus Virtual Machines Memory characteristics (e.g. processor caches) … Other synchronisation points
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That’s hard
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That’s REALLY hard
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That’s REALLY hard Seriously, people spend their entire careers on this
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So, what to do?
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Measure
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Measure Measure
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Measure Measure Measure
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Measure ON PRODUCTION
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Measure ON PRODUCTION You WILL get false results on staging/locally
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Measure Entire system You WILL get false results for single functions
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Measure ONLY IF YOU HAVE TRAFFIC
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“premature optimization is the root of all evil”
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If something takes X ms, it will always take X ms.
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Async execution cannot “remove” this time It can only hide it
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BACK PRESSURE
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Producent Consumer Consumer
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Producent Consumer Consumer
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Producent Consumer Consumer
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Producent Consumer Consumer
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Producent Consumer Consumer
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Producent Consumer Consumer
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Producent Consumer Consumer
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Producent Consumer Consumer
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Producent Consumer Consumer
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Producent Consumer Consumer
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Producent Consumer Consumer
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Producent Consumer Consumer
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Producent Consumer Consumer
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Producent Consumer Consumer Stop
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Producent Consumer Consumer
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Producent Consumer Consumer
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Producent Consumer Consumer
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Producent Consumer Consumer
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Producent Consumer Consumer OK, give me more
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Producent Consumer Consumer
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Back pressure
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Thanks!