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Message-ID: <7c86c4470812150458o47292790gf143fb035bf1e06d@mail.gmail.com>
Date: Mon, 15 Dec 2008 13:58:18 +0100
From: "stephane eranian" <eranian@googlemail.com>
Reply-To: eranian@gmail.com
To: "Paul Mackerras" <paulus@samba.org>
Subject: Re: [patch] Performance Counters for Linux, v3
Cc: "Ingo Molnar" <mingo@elte.hu>, "Peter Zijlstra" <a.p.zijlstra@chello.nl>,
       "Vince Weaver" <vince@deater.net>, linux-kernel@vger.kernel.org,
       "Thomas Gleixner" <tglx@linutronix.de>,
       "Andrew Morton" <akpm@linux-foundation.org>,
       "Eric Dumazet" <dada1@cosmosbay.com>,
       "Robert Richter" <robert.richter@amd.com>,
       "Arjan van de Veen" <arjan@infradead.org>,
       "Peter Anvin" <hpa@zytor.com>, "David S. Miller" <davem@davemloft.net>
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Hi,

On Mon, Dec 15, 2008 at 1:37 AM, Paul Mackerras <paulus@samba.org> wrote:
> Ingo Molnar writes:
>
>> * stephane eranian <eranian@googlemail.com> wrote:
>>
>> > Hi,
>> >
>> > Given the level of abstractions you are using for the API, and given
>> > your argument that the kernel can do the HW resource scheduling better
>> > than anybody else.
>> >
>> > What happens in the following test case:
>> >
>> >    - 2-way system (cpu0, cpu1)
>> >
>> >    - on cpu0, two processes P1, P2, each self-monitoring and counting event E1.
>> >      Event E1 can only be measured on counter C1.
>> >
>> >    - on cpu1, there is a cpu-wide session, monitoring event E1, thus using C1
>> >
>> >    - the scheduler decides to migrate P1 onto CPU1. You now have a
>> >      conflict on C1.
>> >
>> > How is this managed?
>>
>> If there's a single unit of sharable resource [such as an event counter,
>> or a physical CPU], then there's just three main possibilities: either
>> user 1 gets it all, or user 2 gets it all, or they share it.
>>
>> We've implemented the essence of these variants, with sharing the resource
>> being the sane default, and with the sysadmin also having a configuration
>> vector to reserve the resource to himself permanently. (There could be
>> more variations of this.)
>>
>> What is your point?
>>
Could you explain what you mean by sharing here?

Are you talking about time multiplexing the counter?