Horton

Re: [PoC] Improve dead tuple storage for lazy vacuum

John Naylor <john.naylor@enterprisedb.com>

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From: John Naylor <john.naylor@enterprisedb.com>
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: Nathan Bossart <nathandbossart@gmail.com>, Andres Freund <andres@anarazel.de>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Yura Sokolov <y.sokolov@postgrespro.ru>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2022-11-21T07:20:03Z
Lists: pgsql-hackers 
On Fri, Nov 18, 2022 at 2:48 PM I wrote:
> One issue with this patch: The "fanout" member is a uint8, so it can't
hold 256 for the largest node kind. That's not an issue in practice, since
we never need to grow it, and we only compare that value with the count in
an Assert(), so I just set it to zero. That does break an invariant, so
it's not great. We could use 2 bytes to be strictly correct in all cases,
but that limits what we can do with the smallest node kind.

Thinking about this part, there's an easy resolution -- use a different
macro for fixed- and variable-sized node kinds to determine if there is a
free slot.

Also, I wanted to share some results of adjusting the boundary between the
two smallest node kinds. In the hackish attached patch, I modified the
fixed height search benchmark to search a small (within L1 cache) tree
thousands of times. For the first set I modified node4's maximum fanout and
filled it up. For the second, I set node4's fanout to 1, which causes 2+ to
spill to node32 (actually the partially-filled node15 size class
as demoed earlier).

node4:

NOTICE:  num_keys = 16, height = 3, n4 = 15, n15 = 0, n32 = 0, n128 = 0,
n256 = 0
 fanout | nkeys | rt_mem_allocated | rt_load_ms | rt_search_ms
--------+-------+------------------+------------+--------------
      2 |    16 |            16520 |          0 |            3

NOTICE:  num_keys = 81, height = 3, n4 = 40, n15 = 0, n32 = 0, n128 = 0,
n256 = 0
 fanout | nkeys | rt_mem_allocated | rt_load_ms | rt_search_ms
--------+-------+------------------+------------+--------------
      3 |    81 |            16456 |          0 |           17

NOTICE:  num_keys = 256, height = 3, n4 = 85, n15 = 0, n32 = 0, n128 = 0,
n256 = 0
 fanout | nkeys | rt_mem_allocated | rt_load_ms | rt_search_ms
--------+-------+------------------+------------+--------------
      4 |   256 |            16456 |          0 |           89

NOTICE:  num_keys = 625, height = 3, n4 = 156, n15 = 0, n32 = 0, n128 = 0,
n256 = 0
 fanout | nkeys | rt_mem_allocated | rt_load_ms | rt_search_ms
--------+-------+------------------+------------+--------------
      5 |   625 |            16488 |          0 |          327


node32:

NOTICE:  num_keys = 16, height = 3, n4 = 0, n15 = 15, n32 = 0, n128 = 0,
n256 = 0
 fanout | nkeys | rt_mem_allocated | rt_load_ms | rt_search_ms
--------+-------+------------------+------------+--------------
      2 |    16 |            16488 |          0 |            5
(1 row)

NOTICE:  num_keys = 81, height = 3, n4 = 0, n15 = 40, n32 = 0, n128 = 0,
n256 = 0
 fanout | nkeys | rt_mem_allocated | rt_load_ms | rt_search_ms
--------+-------+------------------+------------+--------------
      3 |    81 |            16520 |          0 |           28

NOTICE:  num_keys = 256, height = 3, n4 = 0, n15 = 85, n32 = 0, n128 = 0,
n256 = 0
 fanout | nkeys | rt_mem_allocated | rt_load_ms | rt_search_ms
--------+-------+------------------+------------+--------------
      4 |   256 |            16408 |          0 |           79

NOTICE:  num_keys = 625, height = 3, n4 = 0, n15 = 156, n32 = 0, n128 = 0,
n256 = 0
 fanout | nkeys | rt_mem_allocated | rt_load_ms | rt_search_ms
--------+-------+------------------+------------+--------------
      5 |   625 |            24616 |          0 |          199

In this test, node32 seems slightly faster than node4 with 4 elements, at
the cost of more memory.

Assuming the smallest node is fixed size (i.e. fanout/capacity member not
part of the common set, so only part of variable-sized nodes), 3 has a nice
property: no wasted padding space:

node4: 5 + 4+(7) + 4*8 = 48 bytes
node3: 5 + 3     + 3*8 = 32

--
John Naylor
EDB: http://www.enterprisedb.com

Commits

  1. radixtree: Fix SIGSEGV at update of embeddable value to non-embeddable.

  2. Get rid of anonymous struct

  3. Teach radix tree to embed values at runtime

  4. Teach TID store to skip bitmap for small numbers of offsets

  5. Use bump context for TID bitmaps stored by vacuum

  6. Fix alignment of stack variable

  7. Use TidStore for dead tuple TIDs storage during lazy vacuum.

  8. Rethink create and attach APIs of shared TidStore.

  9. Fix inconsistent function prototypes with function definitions.

  10. Fix a calculation in TidStoreCreate().

  11. Fix potential integer handling issue in radixtree.h.

  12. Add TIDStore, to store sets of TIDs (ItemPointerData) efficiently.

  13. Fix link error for test_radixtree module on Windows

  14. Blind attempt to fix ODR violations

  15. Fix incorrect format specifier for int64

  16. Fix redefinition of typedefs

  17. Add template for adaptive radix tree

  18. Fix signedness error in 9f225e992 for gcc

  19. Introduce helper SIMD functions for small byte arrays

  20. Optimize vacuuming of relations with no indexes.

  21. Add bound check before bsearch() for performance

  22. Allocate consecutive blocks during parallel seqscans