linux-kernel - Re: [RFC PATCH 1/3] Unified trace buffer

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Date:	Wed, 24 Sep 2008 12:37:01 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Martin Bligh <mbligh@...gle.com>, 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,
	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, 24 Sep 2008, Linus Torvalds wrote:

> 
> 
> On Wed, 24 Sep 2008, Peter Zijlstra wrote:
> > 
> > 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.
> 
> I would suggest just not allowing page straddling.
> 
> Yeah, it would limit event size to less than a page, but seriously, do 
> people really want more than that? If you have huge events, I suspect it 
> would be a hell of a lot better to support some kind of indirection 
> scheme than to force the ring buffer to handle insane cases.
> 
> Most people will want the events to be as _small_ as humanly possible. The 
> normal event size should hopefully be in the 8-16 bytes, and I think the 
> RFC patch is already broken because it allocates that insane 64-bit event 
> counter for things. Who the hell wants a 64-bit event counter that much? 
> That's broken.

The event counter is just the timestamp (quick patch, simple to fix). The 
term "counter" was bad. It should have been timestamp, which one would 
want a 64bit timestamp. Or at least a way to figure it out. Yes, we can 
store a special event called "timestamp" and have a smaller counter. But 
for simplicity, the 64 bit was easy. The event id was just 16 bits, which 
I think is way more than enough.

The current method has a 16 bit length as well, and prevents crossing of
page boundaries.

Other than that, I would love to have you review more of this patch.

Note, I plan on hacking the "max_event_size", and just have that be the
standard "PAGE_SIZE". If you need a preallocated storage to store events, 
one could just use PAGE_SIZE and fit any event they want into it.

Thanks,

-- Steve

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