Re: Extended Prefetching using Asynchronous IO - proposal and patch

On Thu, May 29, 2014 at 5:23 PM, Heikki Linnakangas
<hlinnakangas@vmware.com> wrote:
> On 05/29/2014 04:12 PM, John Lumby wrote:
>>
>> Thanks for looking at it!
>>
>>> Date: Thu, 29 May 2014 00:19:33 +0300
>>> From: hlinnakangas@vmware.com
>>> To: johnlumby@hotmail.com; pgsql-hackers@postgresql.org
>>> CC: klaussfreire@gmail.com
>>> Subject: Re: [HACKERS] Extended Prefetching using Asynchronous IO -
>>> proposal and patch
>>>
>>> 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.

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.