Folks,
While approving a recent PSARC case (2009/235), a question popped up
with regard to 'optimum MTU value' for a driver that supports Jumbo
frames and I would like to revive that discussion here in the networking
community.
Basic point is:
Configuring jumbo frames is very common among customers. However for
some drivers which support a range of MTU values, the maximum MTU value
might not be the 'optimum MTU' value (because of the way the driver is
implemented). For example, on Neptune (nxge), the maximum is 9000, but
the optimal large MTU is 8150, because of the size of the DMA transfers
the card does in hardware. In short, the optimal large transfer size is
a function of the hardware and the driver
So how do we 'publish' such values?
One school of thought was to document it in performance tunning guide,
manpage for that driver, blogs, whitepaper et al. Issue with this is the
information often gets 'out-dated' (for eg: optimal MTU value itself
changes for a driver) and googling around for such tunables would take
lot of time.
A different school of thought was for each driver to provide such
information through a read-only property, which will be displayed by
'dladm show-linkprop' in a new column or as part of possible values
list, itself.
The more basic issue with both the 'thoughts' above is, MTU is not only
an end-host issue but also a network-wide configuration issue. That is
the administrator must choose an MTU value not only based on local
hardware configuration but also based on the capabilities and
configuration of all the other nodes on his/her L2 network. So how would
specifying one preferred MTU value do good, overall?
thoughts?
thanks
~Girish
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