Horton

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

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2021-07-09T05:37:34Z
Lists: pgsql-hackers 
Hi,

On 2021-07-08 20:53:32 -0700, Andres Freund wrote:
> On 2021-07-07 20:46:38 +0900, Masahiko Sawada wrote:
> > 1. Don't allocate more than 1GB. There was a discussion to eliminate
> > this limitation by using MemoryContextAllocHuge() but there were
> > concerns about point 2[1].
> >
> > 2. Allocate the whole memory space at once.
> >
> > 3. Slow lookup performance (O(logN)).
> >
> > I’ve done some experiments in this area and would like to share the
> > results and discuss ideas.
>
> Yea, this is a serious issue.
>
>
> 3) could possibly be addressed to a decent degree without changing the
> fundamental datastructure too much. There's some sizable and trivial
> wins by just changing vac_cmp_itemptr() to compare int64s and by using
> an open coded bsearch().

Just using itemptr_encode() makes array in test #1 go from 8s to 6.5s on my
machine.

Another thing I just noticed is that you didn't include the build times for the
datastructures. They are lower than the lookups currently, but it does seem
like a relevant thing to measure as well. E.g. for #1 I see the following build
times

array    24.943 ms
tbm     206.456 ms
intset   93.575 ms
vtbm    134.315 ms
rtbm    145.964 ms

that's a significant range...


Randomizing the lookup order (using a random shuffle in
generate_index_tuples()) changes the benchmark results for #1 significantly:

        shuffled time    unshuffled time
array    6551.726 ms      6478.554 ms
intset  67590.879 ms     10815.810 ms
rtbm    17992.487 ms      2518.492 ms
tbm       364.917 ms       360.128 ms
vtbm    12227.884 ms      1288.123 ms



FWIW, I get an assertion failure when using an assertion build:

#2  0x0000561800ea02e0 in ExceptionalCondition (conditionName=0x7f9115a88e91 "found", errorType=0x7f9115a88d11 "FailedAssertion", 
    fileName=0x7f9115a88e8a "rtbm.c", lineNumber=242) at /home/andres/src/postgresql/src/backend/utils/error/assert.c:69
#3  0x00007f9115a87645 in rtbm_add_tuples (rtbm=0x561806293280, blkno=0, offnums=0x7fffdccabb00, nitems=10) at rtbm.c:242
#4  0x00007f9115a8363d in load_rtbm (rtbm=0x561806293280, itemptrs=0x7f908a203050, nitems=10000000) at bdbench.c:618
#5  0x00007f9115a834b9 in rtbm_attach (lvtt=0x7f9115a8c300 <LVTestSubjects+352>, nitems=10000000, minblk=2139062143, maxblk=2139062143, maxoff=32639)
    at bdbench.c:587
#6  0x00007f9115a83837 in attach (lvtt=0x7f9115a8c300 <LVTestSubjects+352>, nitems=10000000, minblk=2139062143, maxblk=2139062143, maxoff=32639)
    at bdbench.c:658
#7  0x00007f9115a84190 in attach_dead_tuples (fcinfo=0x56180322d690) at bdbench.c:873

I assume you just inverted the Assert(found) assertion?

Greetings,

Andres Freund

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