Re: [patch] Performance Counters for Linux, v3

["stephane eranian" <eranian@googlemail.com>]

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?