On 05/29/2014 11:34 PM, Claudio Freire wrote:
On Thu, May 29, 2014 at 5:23 PM, Heikki Linnakangas
<hlinnakan...@vmware.com> wrote:
On 05/29/2014 04:12 PM, John Lumby wrote:


On 05/28/2014 11:52 PM, John Lumby wrote:

The patch seems to assume that you can put the aiocb struct in shared
memory, initiate an asynchronous I/O request from one process, and wait
for its completion from another process. I'm pretty surprised if that
works on any platform.

It works on linux.    Actually this ability allows the asyncio
implementation to
reduce complexity in one respect (yes I know it looks complex enough) :
it makes waiting for completion of an in-progress IO simpler than for
the existing synchronous IO case,.   since librt takes care of the
waiting.
specifically,   no need for extra wait-for-io control blocks
such as in bufmgr's  WaitIO()

[checks]. No, it doesn't work. See attached test program.

It kinda seems to work sometimes, because of the way it's implemented in
glibc. The aiocb struct has a field for the result value and errno, and when
the I/O is finished, the worker thread fills them in. aio_error() and
aio_return() just return the values of those fields, so calling aio_error()
or aio_return() do in fact happen to work from a different process.
aio_suspend(), however, is implemented by sleeping on a process-local mutex,
which does not work from a different process.

Even if it worked on Linux today, it would be a bad idea to rely on it from
a portability point of view. No, the only sane way to make this work is that
the process that initiates an I/O request is responsible for completing it.
If another process needs to wait for an async I/O to complete, we must use
some other means to do the waiting. Like the io_in_progress_lock that we
already have, for the same purpose.

But calls to it are timeouted by 10us, effectively turning the thing
into polling mode.

We don't want polling... And even if we did, calling aio_suspend() in a way that's known to be broken, in a loop, is a pretty crappy way of polling.

Which is odd now that I read carefully, is how come 256 waits of 10us
amounts to anything? That's just 2.5ms, not enough IIUC for any normal
I/O to complete

Wild guess: the buffer you're reading is already in OS cache.

- Heikki


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