Re: MultiXact\SLRU buffers configuration

Good day, all.

I did benchmark of patch on 2 socket Xeon 5220 CPU @ 2.20GHz .
I used "benchmark" used to reproduce problems with SLRU on our
customers setup.
In opposite to Shawn's tests I concentrated on bad case: a lot
of contention.

slru-funcs.sql - function definitions
  - functions creates a lot of subtrunsactions to stress subtrans
  - and select random rows for share to stress multixacts

slru-call.sql - function call for benchmark

slru-ballast.sql - randomly select 1000 consequent rows
    "for update skip locked" to stress multixacts

patch1 - make SLRU buffers configurable
patch2 - make "8-associative banks"

Benchmark done by pgbench.
Inited with scale 1 to induce contention.
    pgbench -i -s 1 testdb

Benchmark 1:
- low number of connections (50), 60% slru-call, 40% slru-ballast
    pgbench -f slru-call.sql@60 -f slru-ballast.sql@40 -c 50 -j 75 -P 1 -T 30 testdb

version | subtrans | multixact | tps
        | buffers  | offs/memb | func+ballast
--------+----------+-----------+------
master  | 32       | 8/16      | 184+119
patch1  | 32       | 8/16      | 184+119
patch1  | 1024     | 8/16      | 121+77
patch1  | 1024     | 512/1024  | 118+75
patch2  | 32       | 8/16      | 190+122
patch2  | 1024     | 8/16      | 190+125
patch2  | 1024     | 512/1024  | 190+127

As you see, this test case degrades with dumb increase of
SLRU buffers. But use of "hash table" in form of "associative
buckets" makes performance stable.

Benchmark 2:
- high connection number (600), 98% slru-call, 2% slru-ballast
    pgbench -f slru-call.sql@98 -f slru-ballast.sql@2 -c 600 -j 75 -P 1 -T 30 testdb

I don't paste "ballast" tps here since 2% make them too small,
and they're very noisy.

version | subtrans | multixact | tps
        | buffers  | offs/memb | func
--------+----------+-----------+------
master  | 32       | 8/16      | 13
patch1  | 32  
    | 8/16      | 13
patch1  | 1024     | 8/16      | 31
patch1  | 1024     | 512/1024  | 53
patch2  | 32       | 8/16      | 12
patch2  | 1024     | 8/16      | 34
patch2  | 1024     | 512/1024  | 67

In this case simple buffer increase does help. But "buckets"
increase performance gain.

I didn't paste here results third part of patch ("Pack SLRU...")
because I didn't see any major performance gain from it, and
it consumes large part of patch diff.

Rebased versions of first two patch parts are attached.

regards,

Yura Sokolov