Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Tomas Vondra <tomas.vondra@enterprisedb.com>

View thread View on postgr.es
From: Tomas Vondra <tomas.vondra@enterprisedb.com>
To: Thomas Munro <thomas.munro@gmail.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: 2024-03-29T23:40:01Z
Lists: pgsql-hackers 
On 3/29/24 22:39, Thomas Munro wrote:
> ...
> 
>> I don't recall seeing a system with disabled readahead, but I'm
>> sure there are cases where it may not really work - it clearly can't
>> work with direct I/O, ...
> 
> Right, for direct I/O everything is slow right now including seq scan.
> We need to start asynchronous reads in the background (imagine
> literally just a bunch of background "I/O workers" running preadv() on
> your behalf to get your future buffers ready for you, or equivalently
> Linux io_uring).  That's the real goal of this project: restructuring
> so we have the information we need to do that, ie teach every part of
> PostgreSQL to predict the future in a standard and centralised way.
> Should work out better than RA heuristics, because we're not just
> driving in a straight line, we can turn corners too.
> 
>> ... but I've also not been very successful with
>> prefetching on ZFS.
> 
> posix_favise() did not do anything in OpenZFS before 2.2, maybe you
> have an older version?
> 

Sorry, I meant the prefetch (readahead) built into ZFS. I may be wrong
but I don't think the regular RA (in linux kernel) works for ZFS, right?

I was wondering if we could use this (posix_fadvise) to improve that,
essentially by issuing fadvise even for sequential patterns. But now
that I think about that, if posix_fadvise works since 2.2, maybe RA
works too now?)


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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.