Re: Extended Prefetching using Asynchronous IO - proposal and patch

On Mon, Jun 23, 2014 at 2:43 PM, John Lumby <johnlumby@hotmail.com> wrote:
> It is when some *other* backend gets there first with the ReadBuffer that
> things are a bit trickier.    The current version of the patch did polling for that case
> but that drew criticism,    and so an imminent new version of the patch
> uses the sigevent mechanism.    And there are other ways still.

I'm a bit puzzled by this though. Postgres *already* has code for this
case. When you call ReadBuffer you set the bits on the buffer
indicating I/O is in progress. If another backend does ReadBuffer for
the same block they'll get the same buffer and then wait until the
first backend's I/O completes. ReadBuffer goes through some hoops to
handle this (and all the corner cases such as the other backend's I/O
completing and the buffer being reused for another block before the
first backend reawakens). It would be a shame to reinvent the wheel.

The problem with using the Buffers I/O in progress bit is that the I/O
might complete while the other backend is busy doing stuff. As long as
you can handle the I/O completion promptly -- either in callback or
thread or signal handler then that wouldn't matter. But I'm not clear
that any of those will work reliably.

-- 
greg