Horton

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

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: John Naylor <johncnaylorls@gmail.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>, Nathan Bossart <nathandbossart@gmail.com>, Andres Freund <andres@anarazel.de>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Yura Sokolov <y.sokolov@postgrespro.ru>
Date: 2024-01-31T05:49:21Z
Lists: pgsql-hackers

Attachments

On Tue, Jan 30, 2024 at 7:20 PM John Naylor <johncnaylorls@gmail.com> wrote:
>
> On Tue, Jan 30, 2024 at 7:56 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > On Mon, Jan 29, 2024 at 8:48 PM John Naylor <johncnaylorls@gmail.com> wrote:
>
> > > I meant the macro could probably be
> > >
> > > Max(SLAB_DEFAULT_BLOCK_SIZE, (size) * N)
> > >
> > > (Right now N=32). I also realize I didn't answer your question earlier
> > > about block sizes being powers of two. I was talking about PG in
> > > general -- I was thinking all block sizes were powers of two. If
> > > that's true, I'm not sure if it's because programmers find the macro
> > > calculations easy to reason about, or if there was an implementation
> > > reason for it (e.g. libc behavior). 32*2088 bytes is about 65kB, or
> > > just above a power of two, so if we did  round that up it would be
> > > 128kB.
> >
> > Thank you for your explanation. It might be better to follow other
> > codes. Does the calculation below make sense to you?
> >
> > RT_SIZE_CLASS_ELEM size_class = RT_SIZE_CLASS_INFO[i];
> > Size inner_blocksize = SLAB_DEFAULT_BLOCK_SIZE;
> > while (inner_blocksize < 32 * size_class.allocsize)
> >      inner_blocksize <<= 1;
>
> It does make sense, but we can do it more simply:
>
> Max(SLAB_DEFAULT_BLOCK_SIZE, pg_nextpower2_32(size * 32))

Thanks!

I've attached the new patch set (v56). I've squashed previous updates
and addressed review comments on v55 in separate patches. Here are the
update summary:

0004: fix compiler warning caught by ci test.
0005-0008: address review comments on radix tree codes.
0009: cleanup #define and #undef
0010: use TEST_SHARED_RT macro for shared radix tree test. RT_SHMEM is
undefined after including radixtree.h so we should not use it in test
code.
0013-0015: address review comments on tidstore codes.
0017-0018: address review comments on vacuum integration codes.

Looking at overall changes, there are still XXX and TODO comments in
radixtree.h:

---
 * XXX There are 4 node kinds, and this should never be increased,
 * for several reasons:
 * 1. With 5 or more kinds, gcc tends to use a jump table for switch
 *    statements.
 * 2. The 4 kinds can be represented with 2 bits, so we have the option
 *    in the future to tag the node pointer with the kind, even on
 *    platforms with 32-bit pointers. This might speed up node traversal
 *    in trees with highly random node kinds.
 * 3. We can have multiple size classes per node kind.

Can we just remove "XXX"?

---
 * WIP: notes about traditional radix tree trading off span vs height...

Are you going to write it?

---
#ifdef RT_SHMEM
/*  WIP: do we really need this? */
typedef dsa_pointer RT_HANDLE;
#endif

I think it's worth having it.

---
 * WIP: The paper uses at most 64 for this node kind. "isset" happens to fit
 * inside a single bitmapword on most platforms, so it's a good starting
 * point. We can make it higher if we need to.
 */
#define RT_FANOUT_48_MAX (RT_NODE_MAX_SLOTS / 4)

Are you going to work something on this?

---
    /* WIP: We could go first to the higher node16 size class */
    newnode = RT_ALLOC_NODE(tree, RT_NODE_KIND_16, RT_CLASS_16_LO);

Does it mean to go to RT_CLASS_16_HI and then further go to
RT_CLASS_16_LO upon further deletion?

---
 * TODO: The current locking mechanism is not optimized for high concurrency
 * with mixed read-write workloads. In the future it might be worthwhile
 * to replace it with the Optimistic Lock Coupling or ROWEX mentioned in
 * the paper "The ART of Practical Synchronization" by the same authors as
 * the ART paper, 2016.

I think it's not TODO for now, but a future improvement. We can remove it.

---
/* TODO: consider 5 with subclass 1 or 2. */
#define RT_FANOUT_4     4

Is there something we need to do here?

---
/*
 * Return index of the chunk and slot arrays for inserting into the node,
 * such that the chunk array remains ordered.
 * TODO: Improve performance for non-SIMD platforms.
 */

Are you going to work on this?

---
/* Delete the element at 'idx' */
/*  TODO: replace slow memmove's */

Are you going to work on this?

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.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