linux-kernel - Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

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Date:	Mon, 18 Jan 2010 14:15:57 -0800
From:	Jim Keniston <jkenisto@...ibm.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Avi Kivity <avi@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>, ananth@...ibm.com,
	Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	utrace-devel <utrace-devel@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Maneesh Soni <maneesh@...ibm.com>,
	Mark Wielaard <mjw@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

On Mon, 2010-01-18 at 14:57 +0200, Pekka Enberg wrote:
> On 01/18/2010 02:51 PM, Pekka Enberg wrote:
> >> And how many probes do we expected to be live at the same time in
> >> real-world scenarios? I guess Avi's "one million" is more than enough?
> 
> Avi Kivity kirjoitti:
> > I don't think a user will ever come close to a million, but we can 
> > expect some inflation from inlined functions (I don't know if uprobes 
> > replicates such probes, but if it doesn't, it should).
> 
> Right. I guess we're looking at few megabytes of the address space for 
> normal scenarios which doesn't seem too excessive.
> 
> However, as Peter pointed out, the bigger problem is that now we're 
> opening the door for other features to steal chunks of the address 
> space. And I think it's a legitimate worry that it's going to cause 
> problems for 32-bit in the future.
> 
> I don't like the idea but if the performance benefits are real (are 
> they?),

Based on what seems to be the closest thing to an apples-to-apples
comparison -- counting the number of calls to a specified function --
uprobes is 6-7 times faster than the ptrace-based equivalent, ltrace -c.
And of course, uprobes provides much, much more flexibility, appears to
scale better, and works with multithreaded apps.

Likewise, FWIW, utrace is more than 10x faster than strace -c in
counting system calls.

> maybe it's a worthwhile trade-off. Dunno.
> 
> 			Pekka

Jim

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