Re: AIO v2.3 - Mailing list pgsql-hackers

Hi,

On 2025-02-12 13:00:22 -0500, Robert Haas wrote:
> On Tue, Feb 11, 2025 at 4:43 PM Andres Freund <andres@anarazel.de> wrote:
> > One way we could avoid the need for a mechanism to reset-batch-in-progress
> > would be to make batch submission controlled by a flag on the IO. Something
> > like
> >     pgaio_io_set_flag(ioh, PGAIO_HF_BATCH_SUBMIT)
> >
> > IFF PGAIO_HF_BATCH_SUBMIT is set, the IOs would need to be explicitly
> > submitted using something like the existing
> >     pgaio_submit_staged();
> > (although renaming it to something with batch in the name might be
> > appropriate)
> >
> > That way there's no explicit "we are in a batch" state that needs to be reset
> > in case of errors.
> 
> I'll defer to Thomas or others on whether this is better or worse,
> because I don't know. It means that the individual I/Os have to know
> that they are in a batch, which isn't necessary with the begin/end
> batch interface. But if we're expecting that to happen in a pretty
> confined amount of code -- similar to WAL record construction -- then
> that might not be a problem anyway.
> 
> I think if you don't do this, I'd do (sub)xact callbacks rather than a
> resowner integration, unless you decide you want to support multiple
> concurrent batches. You don't really need or want to tie it to a
> resowner unless there are multiple objects each of which can have its
> own resources.

I have working code for that. There unfortunately is an annoying problem:

It afaict is not really possible to trigger a WARNING/ERRROR/Assert in xact
callbacks at the end of commands in an explicit transaction.  Consider
something like:

  BEGIN;
  SELECT start_but_not_end_aio_batch();

we would like to flag that the SELECT query started an AIO batch but didn't
end it. But at the momment there, afaict, is no proper way to do that, because
xact.c won't actually do much at the end of a command in a transaction block:

            /*
             * This is the case when we have finished executing a command
             * someplace within a transaction block.  We increment the command
             * counter and return.
             */
        case TBLOCK_INPROGRESS:
        case TBLOCK_IMPLICIT_INPROGRESS:
        case TBLOCK_SUBINPROGRESS:
            CommandCounterIncrement();
            break;

If one instead integrates with resowners, that kind of thing works, because
exec_simple_query() calls PortalDrop(), which in turn calls
ResourceOwnerRelease().


And we don't reliably warn at a later time. While xact.c integration triggers
a warning for:

  BEGIN;
  SELECT start_but_not_end_aio_batch();
  COMMIT;

as we'd still have the batch open at COMMIT, it wouldn't trigger for

  BEGIN;
  SELECT start_but_not_end_aio_batch();
  ROLLBACK;

as AbortTransaction() might be called in an error and therefore can't assume
that it's a problem for a batch to still be open.


I guess I could just put something alongside that CommandCounterIncrement()
call, but that doesn't seem right.  I guess putting it alongside the
ResourceOwnerRelease() in PortalDrop() is a bit less bad? But still doesn't
seem great.


Just using resowners doesn't seem right either, it's not really free to
register something with resowners, and for read intensive IO we can start a
*lot* of batches, so doing unnecessary work isn't great.

Greetings,

Andres Freund