Subject: Re: [ANNOUNCE] New utility: 'trace'
From: Steven Rostedt <rostedt@goodmis.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>, "Ted Ts'o" <tytso@mit.edu>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Arjan van de Ven <arjan@infradead.org>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Tom Zanussi <tzanussi@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Li Zefan <lizf@cn.fujitsu.com>, Jason Baron <jbaron@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Christoph Hellwig <hch@lst.de>, Pekka Enberg <penberg@kernel.org>,
        Lai Jiangshan <laijs@cn.fujitsu.com>,
        Eric Dumazet <eric.dumazet@gmail.com>
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Date: Wed, 17 Nov 2010 11:15:21 -0500
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On Wed, 2010-11-17 at 16:43 +0100, Peter Zijlstra wrote:
> On Wed, 2010-11-17 at 10:10 -0500, Steven Rostedt wrote:
> > 
> > Right, the problem with filtering is what do we want to filter, and what
> > about copying?
> > 
> > Currently, we copy the data into the buffer and then filter on that
> > data. We could also easily filter on the parameters of the tracepoint,
> > but sometimes those parameters do not match the final output (as the
> > case with sched_switch). Do we copy the data into a separate "per cpu"
> > temp buffer, and figure out the filter then? And if the filter is fine,
> > then copy into the buffer. This obviously is slow, due to the multiple
> > copies. We could do this only if the filtering is enabled.
> 
> Right, so what is the primary purpose of this filtering stuff? As it
> stands it makes stuff terribly slow, so you add overhead but the win
> (presumably) is less data output, is that a sane trade-off?

I've actually used filtering too. Not for speed up, but because I was
recording a lot of data and the reader could not keep up. By filtering,
I was able to get all the relevant information without needing to make
the kernel buffer a Gig.

-- Steve


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