[PATCH] 2PC state files on shared memory - Mailing list pgsql-hackers

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