Hi Charles,
thank you very much for the PX traces. My assumption about some kind of
adaptive feature was right as you can see in the trace file ("adaptive=on").
-----------------8<---------------------------
2015-07-15 15:35:19.992634*:PX_Messaging:kxfp.c@9923:kxfpgsg(begin):
reqthreads=100 height=0 lsize=0 alloc_flg=0x230
2015-07-15 15:35:19.992634*:PX_Messaging:kxfp.c@9996:kxfpgsg():
reqthreads=100 KXFPLDBL/KXFPADPT/ load balancing=on adaptive=on
-----------------8<---------------------------
However your assumption about parallel_degree_limit=CPU and
parallel_degree_policy=MANUAL can not be true imo as parallel_degree_limit is
used for
AutoDOP. In addition in your first case you would limit it to 2
(PARALLEL_THREADS_PER_CPU x CPU_COUNT x number of instances available) in any
case.
This also fits to your study (parallel_degree_policy=AUTO +
parallel_degree_limit=100) in the second test case.
In your initial case (BANIMP_ora_25263.trc) i would count on
parallel_adaptive_multi_user. Can you test it by setting only parameter
parallel_adaptive_multi_user to FALSE? Unfortuantely the exact algorithm is not
known (or at least i never have found anything in great detail about
it).
Best Regards
Stefan Koehler
Freelance Oracle performance consultant and researcher
Homepage: http://www.soocs.de
Twitter: @OracleSK
Charles Schultz <sacrophyte@xxxxxxxxx> hat am 16. Juli 2015 um 16:58
geschrieben:
I think I finally got to the bottom of this - running more tests to confirm.
It seems like, due to parallel_degree_limit=CPU,
parallel_degree_policy=MANUAL, and the default DOP being so low (2), the
kernel decided the CPU was too loaded to grant the requested DOP, so
instead calcuated that a DOP of 2 would be easier on the CPU. Unfortunately,
while this is indeed true, it kills performance.
It seems like using parallel_degree_policy=AUTO and
parallel_degree_limit=100 (or some other high number, maybe even "IO") will
avoid the kernel
freaking out because the CPU has a little load on it, and the added advantage
is that AUTO will defer until more slaves are available. Going to bump
cpu_count=256 for another test, as well.
Hat tip to Anju Garg's blog Oracle In Action
<http://oracleinaction.com/tag/parallel_adaptive_multi_user/> (has several
posts on parallelism).
Thanks again to Jonathan Lewis and Stefan Koehler for helping me get started
on the science and diagnostics.
--
Charles Schultz
