Re: Streaming read-ready sequential scan code

On Fri, Mar 8, 2024 at 4:56 PM Melanie Plageman
<melanieplageman(at)gmail(dot)com> wrote:
>
> On Sat, Mar 02, 2024 at 06:07:48PM -0500, Melanie Plageman wrote:
> > On Wed, Feb 28, 2024 at 12:30 PM Melanie Plageman
> > <melanieplageman(at)gmail(dot)com> wrote:
> > >
> > > On Mon, Feb 26, 2024 at 03:56:57PM -0500, Melanie Plageman wrote:
> > > > On Mon, Feb 19, 2024 at 6:05 PM Melanie Plageman
> > > > <melanieplageman(at)gmail(dot)com> wrote:
> > > > >
> > > > > On Mon, Jan 29, 2024 at 4:17 PM Melanie Plageman
> > > > > <melanieplageman(at)gmail(dot)com> wrote:
> > > > > >
> > > > > > There is an outstanding question about where to allocate the
> > > > > > PgStreamingRead object for sequential scans
> > > > >
> > > > > I've written three alternative implementations of the actual streaming
> > > > > read user for sequential scan which handle the question of where to
> > > > > allocate the streaming read object and how to handle changing scan
> > > > > direction in different ways.
> > > > >
> > > > > Option A) https://github.com/melanieplageman/postgres/tree/seqscan_pgsr_initscan_allocation
> > > > > - Allocates the streaming read object in initscan(). Since we do not
> > > > > know the scan direction at this time, if the scan ends up not being a
> > > > > forwards scan, the streaming read object must later be freed -- so
> > > > > this will sometimes allocate a streaming read object it never uses.
> > > > > - Only supports ForwardScanDirection and once the scan direction
> > > > > changes, streaming is never supported again -- even if we return to
> > > > > ForwardScanDirection
> > > > > - Must maintain a "fallback" codepath that does not use the streaming read API
> > > >
> > > > Attached is a version of this patch which implements a "reset"
> > > > function for the streaming read API which should be cheaper than the
> > > > full pg_streaming_read_free() on rescan. This can easily be ported to
> > > > work on any of my proposed implementations (A/B/C). I implemented it
> > > > on A as an example.
> > >
> > > Attached is the latest version of this patchset -- rebased in light of
> > > Thomas' updatees to the streaming read API [1]. I have chosen the
> > > approach I think we should go with. It is a hybrid of my previously
> > > proposed approaches.
> >
> > While investigating some performance concerns, Andres pointed out that
> > the members I add to HeapScanDescData in this patch push rs_cindex and
> > rs_ntuples to another cacheline and introduce a 4-byte hole. Attached
> > v4's HeapScanDescData is as well-packed as on master and its members
> > are reordered so that rs_cindex and rs_ntuples are back on the second
> > cacheline of the struct's data.
>
> I did some additional profiling and realized that dropping the
> unlikely() from the places we check rs_inited frequently was negatively
> impacting performance. v5 adds those back and also makes a few other
> very minor cleanups.
>
> Note that this patch set has a not yet released version of Thomas
> Munro's Streaming Read API with a new ramp-up logic which seems to fix a
> performance issue I saw with my test case when all of the sequential
> scan's blocks are in shared buffers. Once he sends the official new
> version, I will rebase this and point to his explanation in that thread.

Attached v6 has the version of the streaming read API mentioned here
[1]. This resolved the fully-in-shared-buffers regressions
investigated in that thread by Andres, Bilal, and Thomas.

The one outstanding item for the sequential scan streaming read user
is deciding how the BAS_BULKREAD buffer access strategy should
interact with the streaming read infrastructure. We discussed a bit
off-list, and it seems clear that the ring must be at least as large
as io_combine_limit. This should be no problem for BAS_BULKREAD
because its ring is 16 MB. The question is whether or not we need to
do anything right now to ensure there aren't adverse interactions
between io_combine_limit, max_ios, and the buffer access strategy ring
buffer size.

- Melanie

[1] https://www.postgresql.org/message-id/CA%2BhUKGJTwrS7F%3DuJPx3SeigMiQiW%2BLJaOkjGyZdCntwyMR%3DuAw%40mail.gmail.com