mempool cache change

[Bruce Richardson]

On Tue, Jul 21, 2026 at 11:02:34AM +0200, Morten Brørup wrote:
>    The mempool cache size is configurable.
> 
>    The application developer should configure it to strike a balance
>    between the two performance parameters:
> 
>    - Throughput: Large mempool cache reduces the number of backend
>    transfers.
> 
>    - Latency: Large mempool cache causes latency spikes (CPU cache misses)
>    when transferring objects from/to cold backend memory.
> 
> 
Since 26.11 is an ABI break release, perhaps we can reconsider the option
to have the "keep" value configurable for the mempool caches, i.e. for
random alloc/free's set it to 50% like you have, or 25% as proposed by my
patch, while for pipeline apps it should be set to 0%. Rather than us
constantly arguing^H^H^H discussing what the value should be, we can make
it tunable for the developer?

As an alternative version of that proposal, we can simplify and just have a
couple of predefined simple profiles for 2 or 3 values, one for pipeline
apps, and one or two for run-to-completion, rather than exposing the whole
range of possibilities to the user.

/Bruce