reduce IO when reading processor directories (on startup)

As highlighted during the 26-Nov meeting (JPN), they observed a severe lag when starting up with a large number of processors (eg, 10k+).

changed milestone to %v2012

added confirmed enhancement labels

They could patch around the problem by essentially bypassing fileOperation::lookupAndCacheProcessorsPath entirely. Discussed and analyzed the problem with @Mattijs. The initial patch does solve the immediate problem, but would prevent things like restarting from collated -> uncollated format.

We concluded that the real culprit is the system readdir() (wrapped as Foam::readDir()) being called on all processors when analyzing the names in lookupAndCacheProcessorsPath. As long as we are running without distributed roots (ie, shared filesystem) we can call the readdir on the master only and redistribute the names to the sub-procs. The parsing for processor directories:

^processor(?:(\d+)|(?:s(\d+)(?:_(\d+)-(\d+))?))$

is a simple string check and can be applied on all processors.

Updates for @Azami

made the issue visible to everyone

made the issue confidential

mentioned in commit 627d79db

mentioned in commit 249314c0

made the issue visible to everyone

Hi @Azami - I finalized the changes that I believe will have the same effect as your suggestions, but fixed in a different place. As mentioned in the comment above, we wish to retain the ability to change between different output formats (eg, when converting results or restarting from a different method).

To enable quick testing/confirmation from your side, I have made an additional commit (249314c0 or branch issue-1946-fileOperations-v1812) as a backport with just that particular change for v1812. You only need the fileOperation.C file.

Please let us know if our changes also resolve the issues, or if we need to adjust them.

See note on #1953 (closed)

mentioned in commit 3f55626c59799d222081dc848fcdc250f3013761

mentioned in commit 4f50e605

mentioned in issue #2027 (closed)

mentioned in commit 201f117f

Closing out with update starting times. Thank you @Azami!

• channelflow 24M　PCG-DIC (fixed {p}maxIter=300)
• initTime(starting up time : elapse(sec)) of rank0

config	3072 procs	6144 procs	12288 procs	24576 procs
1812	212	729	2441	-
1812 (RIST patch)	7	27	41	-
v2012(uncollated)	-	-	33	80

closed