On Tue, 1 Aug 2006, Andrew Gallatin wrote:
> - The ifnet send queue is a separately locked object from the device driver,
> meaning that for a single enqueue/dequeue pair, we pay an extra four lock
> operations (two for insert, two for remove) per packet.
Going forward, especially now that we support sun4v CoolThreads hardware,
we're going to want to rethink the "single lock" per transmit routine model
that most drivers have. The most expensive operation in transmit routines
is bus_dmamap_load_mbuf_sg(), especially when there is an IOMMU involved
(like on CoolThreads machines) and there is no reason why this needs to be
called with a driver's transmit lock held. I have hard data (from Solaris)
about how much fine grained locking in a 10GbE driver's transmit routine
helps.
Right now, with the exception of locking for the ifnet dispatch queue, I
believe our ifnet API pretty much leaves decisions about the nature and
granularity of synchronization to the device driver author. The ifnet queue
is high on my list to address (hence this thread) -- are there any other parts
of our device driver framework that stand in the way from a device driver
being modified to support greater parallelism in sending?
Robert N M Watson
Computer Laboratory
University of Cambridge
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