[PATCH] 2PC state files on shared memory

Hi all,

Based on an idea of Heikki Linnakangas, here is a patch in order to improve
2PC
by sending the state files of prepared transactions to shared memory instead
of disk.
It is not possible to avoid the Xlog flush operation but reducing the amout
of data sent to disk permits to accelerate 2PC process.

During a checkpoint, only the state files of prepared but not committed
transactions are flushed to disk from shared memory.
The shared memory allocated for state files on shmem is made with an
additionnal parameter called max_state_file_space in postgresql.conf.
Of course if there are too many transactions and not enough space on shared
memory, state files are sent to disk originally.

By default, the space allocated is set at 0 as max_prepared_transaction is
nul in 8.4.

For some other results, please reference to the wiki page I wrote about this
2PC improvement.
http://wiki.postgresql.org/wiki/2PC_improvement:_state_files_in_shared_memory
This page explains the simulation method for the patch analysis and gathers
the main results.

Here are some of the performance results got by testing the code with a
battery-backedup cache Disk Array with 8 disks in RAID0 configuration.
The four tables below depend on the scale factor at 1 or 100 of pgbench and
if the results are normalized or not.
Normalized results have no unit but pure results are in TX/s.
Tests were made using transaction whose state file sizes are 600B and 712B
via pgbench.
As it is possible to see, the patch permits to improve the transaction flow
by up to 15-18%, what is not negligible.

1) Case scale factor 1, normalized results
State File Size (B) 600 712 Use of 2PC State file
on Shmem State file
on Disk No 2PC State file
on Shmem State file
on Disk No 2PC Pgbench conf Conn Trans Tps1-2 Tps2-2 Tps3-2 Tps1-2 Tps2-2
Tps3-2 2 10000 0.078663793 0 1 0.079653 0 1 5 10000 0.105263158 0 1
0.08438061 0 1 10 10000 0.096105528 0 1 0.07166124 0 1 25 10000 0.106321839
0 1 0.12846154 0 1 35 10000 0.138996139 0 1 0.12106136 0 1 50 10000
0.130278527 0 1 0.14072693 0 1 60 10000 0.133937563 0 1 0.1517094 0 1 70
10000 0.17218543 0 1 0.14913295 0 1 80 10000 0.1775 0 1 0.17786561 0 1 90
10000 0.179806362 0 1 0.15232722 0 1 100 10000 0.182242991 0 1 0.15264798 0
1
2) Case scale factor 1, pure TX/s results
State File Size (B) 600 712 Use of 2PC State file
on Shmem State file
on Disk No 2PC State file
on Shmem State file
on Disk No 2PC Pgbench conf Conn Trans Tps1-2 Tps2-2 Tps3-2 Tps1-2 Tps2-2
Tps3-2 2 10000 1163 1017 2873 1134 1033 2301 5 10000 1263 1077 2844 1213
1072 2743 10 10000 1265 1112 2704 1175 1065 2600 25 10000 1233 1085 2477
1205 1038 2338 35 10000 1220 1040 2335 1169 1023 2229 50 10000 1190 1045
2158 1143 992 2065 60 10000 1151 1018 2011 1111 969 1905 70 10000 1127 971
1877 1067 938 1803 80 10000 1091 949 1749 1021 886 1645 90 10000 1050 920
1643 939 831 1540 100 10000 1012 895 1537 889 791 1433

3) Case scale factor 100, normalized results
State File Size (B) 600 712 Use of 2PC State file
on Shmem State file
on Disk No 2PC State file
on Shmem State file
on Disk No 2PC Pgbench conf Conn Trans Tps1-2 Tps2-2 Tps3-2 Tps1-2 Tps2-2
Tps3-2 2 10000 0.031791908 0 1 0.00426621 0 1 5 10000 0.018481848 0 1
0.03858731 0 1 10 10000 0.049115914 0 1 0.07661017 0 1 25 10000 0.06954612 0
1 0.06117247 0 1 35 10000 0.077677841 0 1 0.05846422 0 1 50 10000
0.059885932 0 1 0.08961303 0 1 60 10000 0.071888412 0 1 0.06997743 0 1 70
10000 0.094007051 0 1 0.03571429 0 1 80 10000 0.078838174 0 1 0.05635838 0 1

4) Case scale factor 100, pure results
State File Size (B) 600 712 Use of 2PC State file
on Shmem State file
on Disk No 2PC State file
on Shmem State file
on Disk No 2PC Pgbench conf Conn Trans Tps1-2 Tps2-2 Tps3-2 Tps1-2 Tps2-2
Tps3-2 2 10000 1113 1058 2788 1147 1142 2314 5 10000 1240 1212 2727 1184
1125 2654 10 10000 1225 1150 2677 1203 1090 2565 25 10000 1218 1123 2489
1176 1104 2281 35 10000 1210 1115 2338 1151 1084 2230 50 10000 1153 1090
2142 1127 1039 2021 60 10000 1126 1059 1991 1083 1021 1907 70 10000 1087
1007 1858 1014 986 1770 80 10000 1046 989 1712 983 944 1636

Regards,

--
Michael Paquier

NTT OSSC