On Mon, May 04, 2009 at 03:15:15PM -0700, Rennie Glen Allen wrote:
> Looking at dtrace.c (in the function dtrace_speculation_commit) it
> looks as if the speculated action printfs are actually formatted at
> probe time.
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.
> It seems apparent to me that if the compiler tagged
> each action printf with a format id, then saved only the printf args
> during probe time and subsequently applied the printf formatting at
> commit; that there would be a significantly reduced probe effect in
> the non-error case (i.e. the case in which the speculation buffer is
> ultimately discarded).
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.
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.
Cheers,
- jonathan
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