Thanks Michael.
Here is my (slightly corrected) version of the script, after I've done a bit of
investment in the dtrace manual (always knew it's powerful, but it's more than
that...).
While it appears to run, and when I tried lowering the limit I started getting
results, I'd appreciate it if you could please explain the actions attached to
the last few probes with the different predicates (the numerous return probes
from creat64).
It looks to me a lot like a conditional program flow (first we calculate the
duration, then we commit() the speculation if duration is > limit and discard()
it otherwise) rather than discrete probes that fire independently. I read the
manual as saying that conditional flow isn't possible in D, but I could have
been wrong. What guarantees that the last free probes will fire in order,
producing an IF-THEN-ELSE logic? If nothing guarantees that - why does the
script work?
Thanks for your help,
- Yaniv
#!/usr/sbin/dtrace -Fs
int limit;
dtrace:::BEGIN
{
limit = 1000000000;
}
syscall::creat64:entry
{
self->spec = speculation();
speculate(self->spec);
self->ts=timestamp;
self->duration = 0;
}
fbt:::entry,
fbt:::return
/self->spec/
{
speculate(self->spec);
}
syscall::creat64:return
/self->spec/
{
speculate(self->spec);
self->duration = timestamp - self->ts;
}
syscall::creat64:return
/self->duration > limit/
{
commit(self->spec);
self->spec = 0;
}
syscall::creat64:return
/self->spec/
{
discard(self->spec);
self->spec = 0;
}
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