Message-Id: <200302280429.h1S4T6513658@cambot.circlemud.org> Subject: Re: tcptrace [OFFTOPIC?] Looking for cheap timestamping... Date: Thu, 27 Feb 2003 20:29:06 -0800 From: Jeremy Elson <jelson@circlemud.org>
uaca@alumni.uv.es writes:
>What about interrupt latency mostly caused by interrupts being disabled?
We haven't had this problem, perhaps because we haven't run our
machines at a particularly high interrupt load. The precision seems
to be limited by the clock resolution more than anything else.
As others have pointed out, some cards might do interrupt coalescing,
which can render timestamps useless.
Timestamping in the card can overcome this problem, although for it to
be useful often you need to find a way to reconcile the card's clock
with your CPU clock. Our software can do this translation for you, if
the card's firmware has some form of sync support. I'd love to find
an off-the-shelf NIC that supports this feature; for some parts of our
testbed, we use NICs with programmable firmware and add the feature
ourselves. (But, for Ethernet, we just timestamp the interrupts and
it works fine.)
Jer
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