Messages in this thread | | | Date | Thu, 10 Dec 2009 16:30:39 -0500 | From | "Frank Ch. Eigler" <> | Subject | Re: [PATCH 2/4] ftrace - add function_duration tracer |
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Hi -
On Thu, Dec 10, 2009 at 09:14:54PM +0100, Ingo Molnar wrote: > [...] > > A few thousand entries in a hash table is really not that big a deal. > Except if it's a high-freq event and the huge hash table is kept in the > CPU cache all the time.
OK. (For reference, an int->int hash table slot costs about 40 bytes, so an L2 cache could carry quite a few of them.)
> Firstly, AFAICS each subsequent systemtap probe for the same event > adds chaining overhead - and then you have to disambiguate back to > the originating script.
Right, but at some point this kind of demultiplexing has to occur somewhere along the line. In practice, chaining a la kprobes or tracepoints is negligible compared to the other costs.
> Secondly, is there a way for a single probe to multiplex its output > to multiple apps? AFAICS that's only possible by running multiple > scripts.
As in having multiple files to write to? There's no easy & direct way to do that right now (beyond unmerged per-cpu files in "bulk" mode). One can have systemtap print data on multiple synthetic /proc/.../ files, but that has other tradeoffs. Or one could demultiplex in user space (for example by prefixing text lines, or using binary records).
> > The message we have received time, after time, after time was > > stronger: that a suitable interpreter was not going to be welcome in > > tree. If this is relaxed (and perhaps even if not), we may prototype > > such a thing in the new year. > > FYI, i suggested this to you 2-3 years ago.
OK, well, I hope that when the time comes, the messages will be less mixed than usual. :)
- FChE
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