Re: [Qemu-devel] [PATCH] block: avoid unnecessary bottom halves

[Stefan Hajnoczi]

On Sat, Mar 28, 2015 at 07:37:18AM +0100, Paolo Bonzini wrote:
> bdrv_aio_* APIs can use coroutines to achieve asynchronicity.  However,
> the coroutine may terminate without having yielded back to the caller
> (for example because of something that invokes a nested event loop,
> or because the coroutine is doing nothing at all).  In this case,
> the bdrv_aio_* API must delay the completion to the next iteration
> of the main loop, because bdrv_aio_* will never invoke the callback
> before returning.
> 
> This can be done with a bottom half, and indeed bdrv_aio_* is always
> using one for simplicity.  It is possible to gain some performance
> (~3%) by avoiding this in the common case.  A new field in the
> BlockAIOCBCoroutine struct is set to true until the first time the
> corotine has yielded to its creator, and completion goes through a
> new function bdrv_co_complete.  If the flag is false, bdrv_co_complete
> invokes the callback immediately.  If it is true, the caller will
> notice that the coroutine has completed and schedule the bottom
> half itself.
> 
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
>  block.c | 43 ++++++++++++++++++++++++++++++++++---------
>  1 file changed, 34 insertions(+), 9 deletions(-)

Reviewed-by: Stefan Hajnoczi <address@hidden>

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