Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Thomas Munro <thomas.munro@gmail.com>

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From: Thomas Munro <thomas.munro@gmail.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Melanie Plageman <melanieplageman@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Andres Freund <andres@anarazel.de>, Pg Hackers <pgsql-hackers@postgresql.org>, Nazir Bilal Yavuz <byavuz81@gmail.com>
Date: 2024-03-29T01:12:48Z
Lists: pgsql-hackers 
On Fri, Mar 29, 2024 at 10:43 AM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
> I think there's some sort of bug, triggering this assert in heapam
>
>   Assert(BufferGetBlockNumber(hscan->rs_cbuf) == tbmres->blockno);

Thanks for the repro.  I can't seem to reproduce it (still trying) but
I assume this is with Melanie's v11 patch set which had
v11-0016-v10-Read-Stream-API.patch.

Would you mind removing that commit and instead applying the v13
stream_read.c patches[1]?  v10 stream_read.c was a little confused
about random I/O combining, which I fixed with a small adjustment to
the conditions for the "if" statement right at the end of
read_stream_look_ahead().  Sorry about that.  The fixed version, with
eic=4, with your test query using WHERE a < a, ends its scan with:

...
posix_fadvise(32,0x28aee000,0x4000,POSIX_FADV_WILLNEED) = 0 (0x0)
pread(32,"\0\0\0\0@4\M-5:\0\0\^D\0\M-x\^A"...,40960,0x28acc000) = 40960 (0xa000)
posix_fadvise(32,0x28af4000,0x4000,POSIX_FADV_WILLNEED) = 0 (0x0)
pread(32,"\0\0\0\0\^XC\M-6:\0\0\^D\0\M-x"...,32768,0x28ad8000) = 32768 (0x8000)
posix_fadvise(32,0x28afc000,0x4000,POSIX_FADV_WILLNEED) = 0 (0x0)
pread(32,"\0\0\0\0\M-XQ\M-7:\0\0\^D\0\M-x"...,24576,0x28ae4000) = 24576 (0x6000)
posix_fadvise(32,0x28b02000,0x8000,POSIX_FADV_WILLNEED) = 0 (0x0)
pread(32,"\0\0\0\0\M^@3\M-8:\0\0\^D\0\M-x"...,16384,0x28aee000) = 16384 (0x4000)
pread(32,"\0\0\0\0\M-`\M-:\M-8:\0\0\^D\0"...,16384,0x28af4000) = 16384 (0x4000)
pread(32,"\0\0\0\0po\M-9:\0\0\^D\0\M-x\^A"...,16384,0x28afc000) = 16384 (0x4000)
pread(32,"\0\0\0\0\M-P\M-v\M-9:\0\0\^D\0"...,32768,0x28b02000) = 32768 (0x8000)

In other words it's able to coalesce, but v10 was a bit b0rked in that
respect and wouldn't do as well at that.  Then if you set
io_combine_limit = 1, it looks more like master, eg lots of little
reads, but not as many fadvises as master because of sequential
access:

...
posix_fadvise(32,0x28af4000,0x2000,POSIX_FADV_WILLNEED) = 0 (0x0) -+
pread(32,...,8192,0x28ae8000) = 8192 (0x2000)                      |
pread(32,...,8192,0x28aee000) = 8192 (0x2000)                      |
posix_fadvise(32,0x28afc000,0x2000,POSIX_FADV_WILLNEED) = 0 (0x0) ---+
pread(32,...,8192,0x28af0000) = 8192 (0x2000)                      | |
pread(32,...,8192,0x28af4000) = 8192 (0x2000) <--------------------+ |
posix_fadvise(32,0x28b02000,0x2000,POSIX_FADV_WILLNEED) = 0 (0x0) -----+
pread(32,...,8192,0x28af6000) = 8192 (0x2000)                        | |
pread(32,...,8192,0x28afc000) = 8192 (0x2000) <----------------------+ |
pread(32,...,8192,0x28afe000) = 8192 (0x2000)  }-- no advice           |
pread(32,...,8192,0x28b02000) = 8192 (0x2000) <------------------------+
pread(32,...,8192,0x28b04000) = 8192 (0x2000)  }
pread(32,...,8192,0x28b06000) = 8192 (0x2000)  }-- no advice
pread(32,...,8192,0x28b08000) = 8192 (0x2000)  }

It becomes slightly less eager to start I/Os as soon as
io_combine_limit > 1, because when it has hit max_ios, if ... <thinks>
yeah if the average block that it can combine is bigger than 4, an
arbitrary number from:

    max_pinned_buffers = Max(max_ios * 4, io_combine_limit);

.... then it can run out of look ahead window before it can reach
max_ios (aka eic), so that's a kind of arbitrary/bogus I/O depth
constraint, which is another way of saying what I was saying earlier:
maybe it just needs more distance.  So let's see the average combined
I/O length in your test query... for me it works out to 27,169 bytes.
But I think there must be times when it runs out of window due to
clustering.  So you could also try increasing that 4->8 to see what
happens to performance.

[1] https://www.postgresql.org/message-id/CA%2BhUKG%2B5UofvseJWv6YqKmuc_%3Drguc7VqKcNEG1eawKh3MzHXQ%40mail.gmail.com

Commits

  1. Fix bitmapheapscan incorrect recheck of NULL tuples

  2. Increase default maintenance_io_concurrency to 16

  3. Separate TBM[Shared|Private]Iterator and TBMIterateResult

  4. Improve read_stream.c advice for dense streams.

  5. Increase default effective_io_concurrency to 16

  6. Delay extraction of TIDBitmap per page offsets

  7. Add lossy indicator to TBMIterateResult

  8. Move BitmapTableScan per-scan setup into a helper

  9. Add and use BitmapHeapScanDescData struct

  10. Fix bitmap table scan crash on iterator release

  11. Bitmap Table Scans use unified TBMIterator

  12. Add common interface for TBMIterators

  13. Make table_scan_bitmap_next_block() async-friendly

  14. Move EXPLAIN counter increment to heapam_scan_bitmap_next_block

  15. Refactor tidstore.c iterator buffering.

  16. BitmapHeapScan: Remove incorrect assert and reset field

  17. Change BitmapAdjustPrefetchIterator to accept BlockNumber

  18. BitmapHeapScan: Use correct recheck flag for skip_fetch

  19. BitmapHeapScan: Push skip_fetch optimization into table AM

  20. BitmapHeapScan: postpone setting can_skip_fetch

  21. BitmapHeapScan: begin scan after bitmap creation

  22. Fix EXPLAIN Bitmap heap scan to count pages with no visible tuples

  23. Remove redundant snapshot copying from parallel leader to workers

  24. Remove some obsolete smgrcloseall() calls.

  25. Remove the "snapshot too old" feature.

  26. Compute XID horizon for page level index vacuum on primary.