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mallopt() allows us to set several env variables to control the behaviour of the malloc implementation.

There doesn't seem to be one to control M_MXFAST, which would be very useful to tune the usage of fast bins without having to recompile the application.

This is not going to get fixed in RHEL6. We don't risk deviate from upstream, and upstream doesn't want to add new environment variables without going through the tunables framework (consistent naming and scope for environment variables that change behaviour). Backporting this new framework to RHEL6 is a nonstarter. Therefore it is likely going to be the case that adjusting fastbin behaviour will always require an application rebuild for RHEL6, while newer RHEL releases will have support for this. As such I'm marking this CLOSED/WONTFIX.