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SubjectRe: [RFC PATCH 1/3] Unified trace buffer


On Wed, 24 Sep 2008, Martin Bligh wrote:
>
> Can't the reserve interface just put a padding event into page A,
> or otherwise mark it, and return the start of page B?

Yes, I think having a "padding" entry that just gets skipped on read would
simplify things. Use that to fill up the end of the page.

> > And here I was thinking you guys bit encoded the event id into the
> > timestamp delta :-)
>
> +/* header plus 32-bits of event data */
> +struct ktrace_entry {
> + u32 event_type:5, tsc_shifted:27;
> + u32 data;
> +};
>
> was our basic data type. So ... sort of ;-)

Why "tsc_shifted"?

I think 27 bits is probably fine, but not by removing precision. Instead
of shifting it so it will fit (and dropping low bits as uninteresting), do
it by encoding it as a delta against the previous thing. 27 bits would
still be sufficient for any high-performance thing that has tons and tons
of packets, and if you only have a few trace events you can afford to have
the "TSC overflow" event type (and if you want it that dense, you could
just make 'data' be the high bits, for a total of 59 bits rather than 64
bits of TSC.

59 bits of cycle counters is perfectly fine unless you are talking trace
events over a year or so (I didn't do the math, but let's assume a 4GHz
TSC as a reasonable thing even going forward - even _if_ CPU's get faster
than that, the TSC is unlikely to tick faster since it's just not worth it
from a power standpoint).

Ok, I did the math. 1<<27 seconds (assuming the low 32 bits are just
fractions) is something like 4+ years. I _really_ don't think we need more
than that (or even close to that) in TSC timestamps for tracing within one
single buffer.

Once you go to the next ring buffer, you'd get a new time-base anyway.

Linus


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