runtime scheduler JBD (@rakyll)
 Google

outline • Introduction • Go’s runtime scheduler • Work stealing and spinning

scheduling? • Controlling the distribution of work over available resources. • Schedulers you might have heard of: process schedulers, network schedulers, container schedulers.

go’s runtime scheduler • Distributes goroutines over multiple OS threads that runs on available processors. • Determine the runnable goroutines, prioritize and run them, park them, and repeat.

processors, threads, goroutines

G1 G1 runnable not runnable G1 running

scheduler • M:N scheduler. • M goroutines need to be scheduled on N OS threads that runs on available processors.

T1 P1 P2 T3 T2 T4 T5 G1 G2 G3 G10 G5 G6 G7 goroutines threads processors runnable queue G8 G4 G9 G0 not runnable goroutines

challenges • Holding a global lock to manage goroutines is costly. • Go is often used to write I/O block heavy programs. Constant preemption of OS threads has significant overhead.

work sharing or stealing? • Work sharing: After creation of new threads, a process attempts to migrate some of them to the other processors. • Work stealing: An underutilized processor actively looks for other processor’s threads and steal some.

T1 P1 P2 T3 T2 T4 T5 G1 G9 G0 G11 G12 G16 goroutines threads processors global queue G2 G3 G4 local queue local queue (empty) steal

work stealing runtime.schedule() {
 // only 1/61 of the time, check the global runnable queue for a goroutine.
 // if not found, check the local queue.
 // if not found,
 // try to steal from other Ps.
 // if not found, check the global runnable queue.
 // if not found, poll network.
 }

spinning • Improve perf both for high throughput and I/O bounded programs. • Prefer to burn some CPU cycles rather than preemption.

spinning: goroutine to run T1 P1 G2 G3 G4 G6 G7 search for runnable goroutines

spinning: processor assignment T1 P1 look for available processors P2 P3

spinning: warm-up T1 P3 (idle) T1 unpark T1 G2 G2 when readying a goroutine

• Scheduler utilizes processing power by scheduling goroutines on the underutilized processors. • Scheduler spins threads to avoid constant preemption.

one more thing… $ go tool trace

not enough concurrency

unbalanced concurrency

thank you JBD (@rakyll)
 jbd@golang.org