linux-kernel - Re: [PATCH] ring_buffer: allocate buffer page pointer

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Message-ID: <alpine.DEB.1.10.0810011342340.2504@gandalf.stny.rr.com>
Date:	Wed, 1 Oct 2008 13:49:59 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Jonathan Corbet <corbet@....net>,
	LKML <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, "Frank Ch. Eigler" <fche@...hat.com>,
	David Wilder <dwilder@...ibm.com>, hch@....de,
	Martin Bligh <mbligh@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH] ring_buffer: allocate buffer page pointer


On Wed, 1 Oct 2008, Mathieu Desnoyers wrote:
> 
> I understand that you want to allocate these struct buffer_page in
> memory local to a given cpu node, which is great, but why do you feel
> you need to align them on cache_line_size() ?
> 
> Hrm.. you put the timestamp in there, so I guess you're concerned about
> having a writer on one CPU, a reader on another, and the fact that you
> will have cache line bouncing because of that.
> 
> Note that if you put the timestamp and the unused bytes in a tiny header
> at the beginning of the page, you
> 
> 1 - make this information directly accessible for disk, network I/O
> without any other abstraction layer.
> 2 - won't have to do such alignment on the struct buffer_page, because
> it will only be read once it's been allocated.
> 

That was the approach I actually started with. But someone (I think
Peter) asked me to remove it.

Who knows, perhaps I can put it back. It's not that hard to do. This is
why I used BUF_PAGE_SIZE to determine the size of the buffer page.
Right now it BUF_PAGE_SIZE == PAGE_SIZE, but if we do add a header than
it will be BUF_PAGE_SIZE == PAGE_SIZE - sizeof(header)

-- Steve

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