It seems that these cause a lot of flushes. Are these chunks of type "freeable"?
The session param values (memory allocated at the start of each session) are large contiguous allocations.
Once the regular pool is chunked up into 4K or smaller pieces large allocations should come from reserved pool.
So, if you get ORA-04031 for an allocation such as this, the solution is usually to increase the size of the reserved pool.
Regards, Dimitre
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