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

On Fri, Sep 23, 2022 at 12:11 AM John Naylor
<john.naylor@enterprisedb.com> wrote:
>
>
> On Thu, Sep 22, 2022 at 11:46 AM John Naylor <john.naylor@enterprisedb.com> wrote:
> > One thing I want to try soon is storing fewer than 16/32 etc entries, so that the whole node fits comfortably
insidea power-of-two allocation. That would allow us to use aset without wasting space for the smaller nodes, which
wouldbe faster and possibly would solve the fragmentation problem Andres referred to in 
>
> > https://www.postgresql.org/message-id/20220704220038.at2ane5xkymzzssb%40awork3.anarazel.de
>
> While calculating node sizes that fit within a power-of-two size, I noticed the current base node is a bit wasteful,
takingup 8 bytes. The node kind only has a small number of values, so it doesn't really make sense to use an enum here
inthe struct (in fact, Andres' prototype used a uint8 for node_kind). We could use a bitfield for the count and kind: 
>
> uint16 -- kind and count bitfield
> uint8 shift;
> uint8 chunk;
>
> That's only 4 bytes. Plus, if the kind is ever encoded in a pointer tag, the bitfield can just go back to being count
only.

Good point, agreed.

>
> Here are the v6 node kinds:
>
> node4:   8 +   4 +(4)    +   4*8 =   48 bytes
> node16:  8 +  16         +  16*8 =  152
> node32:  8 +  32         +  32*8 =  296
> node128: 8 + 256 + 128/8 + 128*8 = 1304
> node256: 8       + 256/8 + 256*8 = 2088
>
> And here are the possible ways we could optimize nodes for space using aset allocation. Parentheses are padding
bytes.Even if my math has mistakes, the numbers shouldn't be too far off: 
>
> node3:   4 +   3 +(1)    +   3*8 =   32 bytes
> node6:   4 +   6 +(6)    +   6*8 =   64
> node13:  4 +  13 +(7)    +  13*8 =  128
> node28:  4 +  28         +  28*8 =  256
> node31:  4 + 256 +  32/8 +  31*8 =  512 (XXX not good)
> node94:  4 + 256 +  96/8 +  94*8 = 1024
> node220: 4 + 256 + 224/8 + 220*8 = 2048
> node256:                         = 4096
>
> The main disadvantage is that node256 would balloon in size.

Yeah, node31 and node256 are bloated.  We probably could use slab for
node256 independently. It's worth trying a benchmark to see how it
affects the performance and the tree size.

BTW We need to consider not only aset/slab but also DSA since we
allocate dead tuple TIDs on DSM in parallel vacuum cases. FYI DSA uses
the following size classes:

static const uint16 dsa_size_classes[] = {
    sizeof(dsa_area_span), 0,   /* special size classes */
    8, 16, 24, 32, 40, 48, 56, 64,  /* 8 classes separated by 8 bytes */
    80, 96, 112, 128,           /* 4 classes separated by 16 bytes */
    160, 192, 224, 256,         /* 4 classes separated by 32 bytes */
    320, 384, 448, 512,         /* 4 classes separated by 64 bytes */
    640, 768, 896, 1024,        /* 4 classes separated by 128 bytes */
    1280, 1560, 1816, 2048,     /* 4 classes separated by ~256 bytes */
    2616, 3120, 3640, 4096,     /* 4 classes separated by ~512 bytes */
    5456, 6552, 7280, 8192      /* 4 classes separated by ~1024 bytes */
};

node256 will be classed as 2616, which is still not good.

Anyway, I'll implement DSA support for radix tree.

Regards,

--
Masahiko Sawada
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com