[Bug 279737] mpi3mr running out of chain in 14.1-RELEASE

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=279737

Muhammad Moinur Rahman <bofh@freebsd.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |Works As Intended
             Status|Open                        |Closed

--- Comment #7 from Muhammad Moinur Rahman <bofh@freebsd.org> ---
(In reply to Sarder Kamal from comment #6)
Definitely someone can help you understand it but it's like a Pandora's Box.
DMA or Direct Memory Access is used by the CPU to reduce the locking of CPU and
so that another device can independently access the Memory rather than slowing
down the CPU. So while making these sort of requests to access the memory the
devices uses chained request rather than sequential requests which are slower
than hell. The problem is with the growth of the CPU markets the OS designers
has to keep up with the number of DMA chained request the OS can handle.
FreeBSD also keeps pushing these limits to a higher value for increased
performance. The more chained DMA requests you can complete that specific piece
of hardware will be able to copy more data from the memory to the devices in
each cycle without locking the CPU. Although the underlying problem is not
within the driver. But now you can find some at least a bit better performance
with(32768) your card although I am not sure if we can benchmark it.

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