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SubjectRe: [RFC PATCH] perf_core: provide a kernel-internal interface to get to performance counters

* Frédéric Weisbecker <fweisbec@gmail.com> wrote:

> 2009/10/5 Ingo Molnar <mingo@elte.hu>:
> >
> > * Peter Zijlstra <peterz@infradead.org> wrote:
> >> Non-trivial.
> >>
> >> Something like this would imply a single output channel for all these
> >> CPUs, and we've already seen that stuffing too many CPUs down one such
> >> channel (using -M) leads to significant performance issues.
> >
> > We could add internal per cpu buffering before it hits any globally
> > visible output channel. (That has come up when i talked to Frederic
> > about the function tracer.) We could even have page sized output
> > (via the introduction of a NOP event that fills up to the next page
> > edge).
>
> That looks good for the counting/sampling fast path, but would that
> scale once it comes to reordering in the globally visible output
> channel? Such a union has its costs.

Well, reordering always has a cost, and we have multiple models
regarding to where to put that cost.

The first model is 'everything is per cpu' - i.e. completely separate
event buffers and the reordering is pushed to the user-space
post-processing stage. This is the most scalable solution - but it can
also lose information such as the true ordering of events.

The second model is 'event multiplexing' - here we use a single output
buffer for events. This serializes all output on the same buffer and
hence is the least scalable one. It is the easiest to use one: just a
single channel of output to deal with. It is also the most precise
solution and it saves the post-processing stage from reordering hassles.

What i suggested above is a third model: 'short-term per cpu,
multiplexed into an output channel with page granularity'. It has the
advantage of being per cpu on a page granular basis. It has the ease of
use of having a single output channel only.

Neither solution can eliminate the costs and tradeoffs involved. What
they do is to offer an app a spectrum to choose from.

Ingo
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