On 05/19/2010 10:39 AM, Rusty Russell wrote:
I think we're talking about the last 2 entries of the avail ring. That means
the worst case is 1 false bounce every time around the ring.
It's low, but why introduce an inefficiency when you can avoid doing it
for the same effort?
I think that's
why we're debating it instead of measuring it :)
Measure before optimize is good for code but not for protocols.
Protocols have to be robust against future changes. Virtio is warty
enough already, we can't keep doing local optimizations.
Note that this is a exclusive->shared->exclusive bounce only, too.
A bounce is a bounce.
Virtio is already way too bouncy due to the indirection between the
avail/used rings and the descriptor pool. A device with out of order
completion (like virtio-blk) will quickly randomize the unused
descriptor indexes, so every descriptor fetch will require a bounce.
In contrast, if the rings hold the descriptors themselves instead of
pointers, we bounce (sizeof(descriptor)/cache_line_size) cache lines for
every descriptor, amortized.
--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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