Re: [PoC] Improve dead tuple storage for lazy vacuum - Mailing list pgsql-hackers

On Mon, Nov 21, 2022 at 4:20 PM John Naylor
<john.naylor@enterprisedb.com> wrote:
>
>
> 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
notan issue in practice, since we never need to grow it, and we only compare that value with the count in an Assert(),
soI just set it to zero. That does break an invariant, so it's not great. We could use 2 bytes to be strictly correct
inall 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
kindsto 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
attachedpatch, 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,
whichcauses 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-sizednodes), 3 has a nice property: no wasted padding space: 
>
> node4: 5 + 4+(7) + 4*8 = 48 bytes
> node3: 5 + 3     + 3*8 = 32

IIUC if we store the fanout member only in variable-sized nodes,
rt_node has only count, shift, and chunk, so 4 bytes in total. If so,
the size of node3 (ie. fixed-sized node) is (4 + 3 + (1) + 3*8)? The
size doesn't change but there is 1 byte padding space.

Also, even if we have the node3 a variable-sized node, size class 1
for node3 could be a good choice since it also doesn't need padding
space and could be a good alternative to path compression.

node3         :  5 + 3 + 3*8 = 32 bytes
size class 1 : 5 + 3 + 1*8 = 16 bytes

Regards,

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