Re: Transactions involving multiple postgres foreign servers, take 2

On 2021/05/11 13:37, Masahiko Sawada wrote:
> I've attached the updated patches that incorporated comments from
> Zhihong and Ikeda-san.

Thanks for updating the patches!

I have other comments including trivial things.

a. about "foreign_transaction_resolver_timeout" parameter

Now, the default value of "foreign_transaction_resolver_timeout" is 60 secs.
Is there any reason? Although the following is minor case, it may confuse some
users.

Example case is that

1. a client executes transaction with 2PC when the resolver is processing
FdwXactResolverProcessInDoubtXacts().

2. the resolution of 1st transaction must be waited until the other
transactions for 2pc are executed or timeout.

3. if the client check the 1st result value, it should wait until resolution
is finished for atomic visibility (although it depends on the way how to
realize atomic visibility.) The clients may be waited
foreign_transaction_resolver_timeout". Users may think it's stale.

Like this situation can be observed after testing with pgbench. Some
unresolved transaction remains after benchmarking.

I assume that this default value refers to wal_sender, archiver, and so on.
But, I think this parameter is more like "commit_delay". If so, 60 seconds
seems to be big value.

b. about performance bottleneck (just share my simple benchmark results)

The resolver process can be performance bottleneck easily although I think
some users want this feature even if the performance is not so good.

I tested with very simple workload in my laptop.

The test condition is
* two remote foreign partitions and one transaction inserts an entry in each
partitions.
* local connection only. If NW latency became higher, the performance became
worse.
* pgbench with 8 clients.

The test results is the following. The performance of 2PC is only 10%
performance of the one of without 2PC.

* with foreign_twophase_commit = requried
-> If load with more than 10TPS, the number of unresolved foreign transactions
is increasing and stop with the warning "Increase
max_prepared_foreign_transactions".

* with foreign_twophase_commit = disabled
-> 122TPS in my environments.

c. v36-0001-Introduce-transaction-manager-for-foreign-transa.patch

* typo: s/tranasction/transaction/

* Is it better to move AtEOXact_FdwXact() in AbortTransaction() to before "if
(IsInParallelMode())" because make them in the same order as CommitTransaction()?

* functions name of fdwxact.c

Although this depends on my feeling, xact means transaction. If this feeling
same as you, the function names of FdwXactRegisterXact and so on are odd to
me. FdwXactRegisterEntry or FdwXactRegisterParticipant is better?

* Are the following better?

- s/to register the foreign transaction by/to register the foreign transaction
participant by/

- s/The registered foreign transactions/The registered participants/

- s/given foreign transaction/given foreign transaction participant/

- s/Foreign transactions involved in the current transaction/Foreign
transaction participants involved in the current transaction/

Regards,

--
Masahiro Ikeda
NTT DATA CORPORATION