spawn_pinned is substantially more expensive than other spawn mechanisms

[futursolo]

Version

• tokio v1.19.2
• tokio-macros v1.8.0 (proc-macro)
• tokio-util v0.7.3
• tokio v1.19.2 (*)

Platform

I have tested the following targets and seems like this behaviour is prevalent on most UNIX platforms.

• aarch64-apple-darwin
• x86_64-apple-darwin
• aarch64-unknown-linux-gnu
• x86_64-unknown-linux-gnu

Description

If LocalPoolHandle::spawn_pinned is used to spawn a future that will be ready immediately (e.g.: async {}), it will take a significantly longer time to complete the task comparing to tokio::task::spawn. However, if a different future that will become temporarily pending is used (e.g.: tokio::time::sleep(Duration::ZERO)), it will be completed with a similar time to tokio::task::spawn.

I wrote a simple test to measure the time used to spawn 1 million tasks and joining them and got the following result:

https://github.com/futursolo/tokio-task-spawn-test/blob/master/src/main.rs

Testing Ready Future...
  Round: 1
    futures ordered: 107ms
        spawn local: 133ms
       spawn pinned: 1221ms
              spawn: 332ms

  Round: 2
    futures ordered: 64ms
        spawn local: 136ms
       spawn pinned: 1402ms
              spawn: 320ms

  Round: 3
    futures ordered: 75ms
        spawn local: 127ms
       spawn pinned: 1127ms
              spawn: 321ms

  Round: 4
    futures ordered: 73ms
        spawn local: 127ms
       spawn pinned: 1455ms
              spawn: 330ms

  Round: 5
    futures ordered: 69ms
        spawn local: 139ms
       spawn pinned: 1447ms
              spawn: 320ms

Testing sleep(Duration::ZERO) Future...
  Round: 1
    futures ordered: 449ms
        spawn local: 474ms
       spawn pinned: 747ms
              spawn: 926ms

  Round: 2
    futures ordered: 467ms
        spawn local: 505ms
       spawn pinned: 715ms
              spawn: 813ms

  Round: 3
    futures ordered: 410ms
        spawn local: 470ms
       spawn pinned: 725ms
              spawn: 833ms

  Round: 4
    futures ordered: 409ms
        spawn local: 478ms
       spawn pinned: 731ms
              spawn: 838ms

  Round: 5
    futures ordered: 399ms
        spawn local: 478ms
       spawn pinned: 727ms
              spawn: 826ms

[Darksonn]

Well, I am not surprised that spawn_pinned is the slowest. I would be happy to accept alternate implementations if they are faster.