linux-kernel - Re: [PATCH v2 4/5] perf record: add meta-data support for pipe-mode

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Message-ID: <1337761273.9698.60.camel@twins>
Date:	Wed, 23 May 2012 10:21:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	David Ahern <dsahern@...il.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu
Subject: Re: [PATCH v2 4/5] perf record: add meta-data support for pipe-mode

On Tue, 2012-05-22 at 19:51 +0200, Stephane Eranian wrote:
> The problem is that the headers as they are written to the file need
> seeking in the file to update the offset table. That is NOT possible when
> you operate in pipe mode. 

I know that, I said as much. But it is quite possible to multiplex
streams without the need for seeking, look at these MKV containers that
contain video, multiple-audio and multiple-subtitle tracks.

The fact is, whomever wrote that pipe mode took a bonghit before
touching the keyboard. They didn't even ask if we could reserve a range
for userspace events, let alone consider if it was a good idea to begin
with.

If we ever manage to get splice() working you can't keep that pipe mode
working anyway.

Pushing everything over a single stream isn't a scalable solution anyway
in the same way that writing everything into the one file isn't the best
thing and would ideally be replaced as well.
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