linux-kernel - Re: [patch 1/2] x86_64 page fault NMI-safe

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Date:	Sun, 15 Aug 2010 12:44:14 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@....de>, Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Johannes Berg <johannes.berg@...el.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Tejun Heo <htejun@...il.com>
Subject: Re: [patch 1/2] x86_64 page fault NMI-safe

* Avi Kivity (avi@...hat.com) wrote:
>  On 08/11/2010 05:34 PM, Steven Rostedt wrote:
>>
>> So, I want to allocate a 10Meg buffer. I need to make sure the kernel
>> has 10megs of memory available. If the memory is quite fragmented, then
>> too bad, I lose out.
>
> With memory compaction, the cpu churns for a while, then you have your  
> buffer.  Of course there's still no guarantee, just a significantly  
> higher probability of success.

The bigger the buffers, the lower the probabilities of success are. My users
often allocate buffers as large as a few GB per cpu. Relying on compaction does
not seem like a viable solution in this case.

>
>> Oh wait, I could also use vmalloc. But then again, now I'm blasting
>> valuable TLB entries for a tracing utility, thus making the tracer have
>> a even bigger impact on the entire system.
>
> Most trace entries will occupy much less than a page, and are accessed  
> sequentially, so I don't think this will have a large impact.

You seem to underestimate the frequency at which trace events can be generated.
E.g., by the time you run the scheduler once (which we can consider a very hot
kernel path), some tracing modes will generate thousands of events, which will
touch a very significant amount of TLB entries.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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