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Message-Id: <1222272686.16700.162.camel@lappy.programming.kicks-ass.net>
Date: Wed, 24 Sep 2008 18:11:25 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Martin Bligh <mbligh@...gle.com>
Cc: Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
Ingo Molnar <mingo@...e.hu>,
Thomas Gleixner <tglx@...utronix.de>,
Andrew Morton <akpm@...ux-foundation.org>,
prasad@...ux.vnet.ibm.com,
Linus Torvalds <torvalds@...ux-foundation.org>,
Mathieu Desnoyers <compudj@...stal.dyndns.org>,
"Frank Ch. Eigler" <fche@...hat.com>,
David Wilder <dwilder@...ibm.com>, hch@....de,
Tom Zanussi <zanussi@...cast.net>,
Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC PATCH 1/3] Unified trace buffer
On Wed, 2008-09-24 at 08:47 -0700, Martin Bligh wrote:
> Thanks for creating this so quickly ;-)
>
> >> We can record either the fast way of reserving a part of the buffer:
> >>
> >> event = ring_buffer_lock_reserve(buffer, event_id, length, &flags);
> >> event->data = record_this_data;
> >> ring_buffer_unlock_commit(buffer, event, flags);
> >
> > This can, in generic, not work. Due to the simple fact that we might
> > straddle a page boundary. Therefore I think its best to limit our self
> > to the write interface below, so that it can handle that.
>
> I'm not sure why this is any harder to deal with in write, than it is
> in reserve? We should be able to make reserve handle this just
> as well?
No, imagine the mentioned case where we're straddling a page boundary.
A----| |----B
^------|
So when we reserve we get a pointer into page A, but our reserve length
will run over into page B. A write() method will know how to check for
this and break up the memcpy to copy up-to the end of A and continue
into B.
You cannot expect the reserve/commit interface users to do this
correctly - it would also require one to expose too much internals,
you'd need to be able to locate page B for starters.
> If you use write rather than reserve, you have to copy all the data
> twice for every event.
Well, once. I'm not seeing where the second copy comes from.
> > On top of that foundation build an eventbuffer, which knows about
> > encoding/decoding/printing events.
> >
> > This too needs to be a flexible layer -
>
> That would be nice. However, we need to keep at least the length
> and timestamp fields common so we can do parsing and the mergesort?
And here I was thinking you guys bit encoded the event id into the
timestamp delta :-)
> +struct ring_buffer_event {
> + unsigned long long counter;
> + short type;
> + short length;
> + char body[];
> +} __attribute__((__packed__))
>
> So type would move into the body here?
All of it would, basically I have no notion of an event in the
ringbuffer API. You write $something and your read routine would need to
be smart enough to figure it out.
The trivial case is a fixed size entry, in which case you always know
how much to read. A slightly more involved but still easy to understand
example might be a 7bit encoding and using the 8th bit for continuation.
Another option is to start out with a fixed sized header that contains a
length field.
But the raw ringbuffer layer, the one concerned with fiddling the pages
and writing/reading thereto need not be aware of anything else.
> > as I suspect the google guys
> > will want their ultra-compressed events back.
>
> Is useful when gathering GB of data across 10,000 machines ;-)
> Also reduces general overhead for everyone to keep events small.
Exactly - which is why a flexible encoding layer makes sense to me -
aside from the abstraction itself.
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