Jonathan,
Thanks! please see in-line...
Rennie Allen,
Systems Engineer, GoAhead Software Inc.
Mobile: (951)704-2447
Email: [email protected]
Web: http://www.goahead.com
Linked-in: http://www.linkedin.com/in/rennieallen
----- Original Message -----
From: Jonathan Adams
Sent: Mon, 5/4/2009 4:17pm
To: [email protected]
Cc: [email protected]
Subject: Re: [dtrace-discuss] speculative tracing
On Mon, May 04, 2009 at 03:15:15PM -0700, Rennie Glen Allen wrote:
> Where, exactly, are you getting this from? All dtrace_speculation_commit()
> does is copy a bunch of data from the speculation buffer to the main buffer.
Yup. I agree, that's all it does.
> I'm not sure what you mean; any printf() action goes from:
> printf("%d %d %d", a, b, c);
>
> to a data buffer that looks like (assuming a, b, c are ints):
> offset
> 0 epid
> 4 a
> 8 b
> c c
> where formatid is an index into the array of formats for this dtrace consumer.
> The epid allows the consumer to get metadata (dtrace_eprobedesc_t) about the
> format of this block, which includes an array of "dtrace_recdesc_t"s,
> describing
> the records.
> The records will be:
> PRINTF (format 1), offset 4, size 4 (a)
> PRINTF (format 2), offset 8, size 4 (b)
> PRINTF (format 3), offset c, size 4 (c)
> Formats 1, 2, and 3 will all reference the string "%d %d %d", and libdtrace
> will only be grab the descriptor of this epid from the kernel once.
Fantastic! That's exactly the way I though it should work (but I wasn't able
to find where this mechanism was coded), in my (too) short investigation.
> It doesn't matter if it's speculative or not; no "formatting" is done until
> the data reaches userland; there's no code in the kernel to even think about
> doing so.
Yes, I didn't think there was a difference between speculative and
non-speculative. I just wasn't able to find the mechanism above in a cursory
glance at the source.
Thanks for taking the time to explain it to me.
Rennie
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