[Bug 1067] linux pmda does too much work for network.interface queries

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Comment # 5 on bug 1067 from Michele Baldessari

One fairly non intrusive and quick ameliorating change (it's not a full fix
obviously) would be to always fetch most/all the interface attributes via
sysfs via the SIOCETHTOOL/SIOC* calls.

At least in Carlos' strace it took one whole second:
23:06:19.435643 ioctl(8, SIOCETHTOOL, 0x7fff7d76d0e0) = 0 <1.038297>

Since SIOCETHTOOL does poke the driver itself, whereas the sysfs attributes
won't, it seems this could be a fairly quick win (at least RHEL 5 has those
files for most interfaces) and should an older kernel not have them
we can always fall back to SIOCETHTOOL.

We could then discuss separately how the linux pmda is fetching too much
data when poked for a few metrics and maybe tackle that in a more intrusive/
architectural way (I have a hunch that network.interface.* is not the only
one). This is likely to at least improve Carlos' situation where a driver might
decide to take a long time to reply to an ethtool poking.

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