test: Add perf test to measure the "cost" of chain wakeup

[xemul]

The test creates a chain of future-promise-s and then wakes up the last one thus causing the cascade of resolutions. There are 2x2 tests -- resolve with value or exception vs co_await-ed or .then()-d chains.

The result is (depth of 32)

test                   iterations      median         mad         min         max      allocs       tasks        inst      cycles
chain.then_value           992498     1.007us     1.715ns     1.001us     1.008us      32.000      33.000      9623.7         0.0
chain.await_value          669230     1.466us     2.124ns     1.457us     1.512us      33.000      34.000     12804.5         0.0
chain.then_exception       955761     1.041us     0.731ns     1.039us     1.044us      34.000      34.000      9906.2         0.0
chain.await_exception        9980    96.767us    57.374ns    96.399us    96.850us      68.000      35.000    747836.2         0.0

Waking up co-await-ed chain with exceptions is extremely expensive.

Worse, if comparing the result with depth of 8

test                   iterations      median         mad         min         max      allocs       tasks        inst      cycles
chain.then_value          3516439   269.729ns     0.459ns   269.243ns   273.017ns       8.000       9.000      2582.4         0.0
chain.await_value         2511108   397.325ns     0.919ns   396.407ns   409.861ns       9.000      10.000      3623.0         0.0
chain.then_exception      2909952   344.783ns     0.559ns   341.926ns   345.653ns      10.000      10.000      3081.4         0.0
chain.await_exception       37663    26.700us   153.423ns    26.422us    27.405us      20.000      11.000    204489.5         0.0

it's clear that exception propagation via co-awaits is expensive on every co_await, as it "scales" linearly with the chain depth.

[avikivity]

The test creates a chain of future-promise-s and then wakes up the last one thus causing the cascade of resolutions. There are 2x2 tests -- resolve with value or exception vs co_await-ed or .then()-d chains.

The result is (depth of 32)

test                   iterations      median         mad         min         max      allocs       tasks        inst      cycles
chain.then_value           992498     1.007us     1.715ns     1.001us     1.008us      32.000      33.000      9623.7         0.0
chain.await_value          669230     1.466us     2.124ns     1.457us     1.512us      33.000      34.000     12804.5         0.0
chain.then_exception       955761     1.041us     0.731ns     1.039us     1.044us      34.000      34.000      9906.2         0.0
chain.await_exception        9980    96.767us    57.374ns    96.399us    96.850us      68.000      35.000    747836.2         0.0

Waking up co-await-ed chain with exceptions is extremely expensive.

Worse, if comparing the result with depth of 8

test                   iterations      median         mad         min         max      allocs       tasks        inst      cycles
chain.then_value          3516439   269.729ns     0.459ns   269.243ns   273.017ns       8.000       9.000      2582.4         0.0
chain.await_value         2511108   397.325ns     0.919ns   396.407ns   409.861ns       9.000      10.000      3623.0         0.0
chain.then_exception      2909952   344.783ns     0.559ns   341.926ns   345.653ns      10.000      10.000      3081.4         0.0
chain.await_exception       37663    26.700us   153.423ns    26.422us    27.405us      20.000      11.000    204489.5         0.0

it's clear that exception propagation via co-awaits is expensive on every co_await, as it "scales" linearly with the chain depth.

Please normalize by the depth, or at least add the depth to the test name. It's not then_value that takes 1 usec, it's 32 of them.