Re: [Qemu-devel] [PATCH] test-coroutine: add baseline test that times the cost of function calls

[Ming Lei]

On Wed, Aug 6, 2014 at 5:33 PM, Paolo Bonzini <address@hidden> wrote:
> This can be used to compute the cost of coroutine operations.  In the
> end the cost of the function call is a few clock cycles, so it's pretty
> cheap for now, but it may become more relevant as the coroutine code
> is optimized.
>
> For example, here are the results on my machine:
>
>    Function call 100000000 iterations: 0.173884 s
>    Yield 100000000 iterations: 8.445064 s
>    Lifecycle 1000000 iterations: 0.098445 s
>    Nesting 10000 iterations of 1000 depth each: 7.406431 s
>
> One yield takes 83 nanoseconds, one enter takes 97 nanoseconds,
> one coroutine allocation takes (roughly, since some of the allocations
> in the nesting test do hit the pool) 739 nanoseconds:
>
>    (8.445064 - 0.173884) * 10^9 / 100000000 = 82.7
>    (0.098445 * 100 - 0.173884) * 10^9 / 100000000 = 96.7
>    (7.406431 * 10 - 0.173884) * 10^9 / 100000000 = 738.9

Thought about it further, the above is _not_ cheap if it is computed correctly,
take block layer for example:

- suppose one block device can reach 300K IOPS, like Kevin's loop
over tmpfs file, so handling one IO takes 3.333us
- for handling one IO, at least two enter and one yield are required in current
implementation, so these three operations take 0.277us(0.083 + 0.097 * 2)
for handling one IO(suppose all allocations hit the pool)
- so coroutine-only operations take 8.31%(0.277/3.333), not mention effect
from switching stack

Some modern storage devices can handle millions of IOPS, so using
coroutine surely will slow down these devices a lot.

Thanks,