Messages in this thread | | | Date | Wed, 24 Sep 2008 11:04:04 -0700 | From | "Martin Bligh" <> | Subject | Re: [RFC PATCH 1/3] Unified trace buffer |
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> If we just record the TSC unshifted, in 27 bits, at 4GHz, that gives us > about 1/30 of a second? So we either shift, use > 27 bits, or record > at least 30 events a second, none of which I like much ...
If we use 32 bits instead of 27, then the timestamp events are only about once per second, which is probably fine for overhead ... ?
I think we're OK losing 5 bits of precision, that's only 32 cycles, given all the CPU reordering stuff we've talked about here, not-quite-synced TSCs, etc. I suspect you thought we were shifting by much more than this, in reality it was 5-10 bits, with timestamp events inbetween, though we put wall time in them, which I think was a mistake.
(note: I'm not suggesting we have to use this compact a format, at least by default)
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