Julian Elischer posted an interesting proof-of-concept to freebsd-current

Julian Elischer posted an interesting proof-of-concept to freebsd-current
regarding UGEN's use of a 1K stack buffer for bulk IO issues.  The small
block size resulted in unnecessarily slow performance with certain devices.

Implement a fix along the lines described.  Create a simple ugen buffer
allocator abstraction and a one-entry cache to avoid unnecessary malloc/free
sequences.  Allow the block size to be set with a sysctl and default it
to 16K.

Not much uses UGEN.  Camera software, mainly.  The change appears to
slightly improve s10sh transfer performance from my Canon 10D.