Speedup twophase transactions

Hello.

While working with cluster stuff (DTM, tsDTM) we noted that postgres 2pc transactions is approximately two times slower
thanan ordinary commit on workload with fast transactions — few single-row updates and COMMIT or PREPARE/COMMIT. Perf
topshowed that a lot of time is spent in kernel on fopen/fclose, so it worth a try to reduce file operations with 2pc
tx.

Now 2PC in postgres does following:
* on prepare 2pc data (subxacts, commitrels, abortrels, invalmsgs) saved to xlog and to file, but file not is not
fsynced
* on commit backend reads data from file
* if checkpoint occurs before commit, then files are fsynced during checkpoint
* if case of crash replay will move data from xlog to files

In this patch I’ve changed this procedures to following:
* on prepare backend writes data only to xlog and store pointer to the start of the xlog record
* if commit occurs before checkpoint then backend reads data from xlog by this pointer
* on checkpoint 2pc data copied to files and fsynced
* if commit happens after checkpoint then backend reads files
* in case of crash replay will move data from xlog to files (as it was before patch)

Most of that ideas was already mentioned in 2009 thread by Michael Paquier
http://www.postgresql.org/message-id/c64c5f8b0908062031k3ff48428j824a9a46f28180ac@mail.gmail.comwhere he suggested to
store2pc data in shared memory.  
At that time patch was declined because no significant speedup were observed. Now I see performance improvements by my
patchat about 60%. Probably old benchmark overall tps was lower and it was harder to hit filesystem fopen/fclose
limits.

Now results of benchmark are following (dual 6-core xeon server):

Current master without 2PC: ~42 ktps
Current master with 2PC: ~22 ktps
Current master with 2PC: ~36 ktps

Benchmark done with following script:

\set naccounts 100000 * :scale
\setrandom from_aid 1 :naccounts
\setrandom to_aid 1 :naccounts
\setrandom delta 1 100
\set scale :scale+1
BEGIN;
UPDATE pgbench_accounts SET abalance = abalance - :delta WHERE aid = :from_aid;
UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE aid = :to_aid;
PREPARE TRANSACTION ':client_id.:scale';
COMMIT PREPARED ':client_id.:scale';



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Stas Kelvich
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company