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

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Date:	Sun, 15 Aug 2010 19:51:15 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.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

  On 08/15/2010 07:44 PM, Mathieu Desnoyers wrote:
> * 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.

Wow.  Even if you could compact that much memory, it would take quite a 
bit of time.

>>> 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.

Let's say a trace entry occupies 40 bytes and a TLB miss costs 200 
cycles on average.  So we have 100 entries per page costing 200 cycles; 
amortized each entry costs 2 cycles.

There's an additional cost caused by the need to re-fill the TLB later, 
but you incur that anyway if the scheduler caused a context switch.

Of course, my assumptions may be completely off (likely larger entries 
but smaller miss costs).  Has a vmalloc based implementation been 
tested?  It seems so much easier than the other alternatives.

-- 
error compiling committee.c: too many arguments to function

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