Messages in this thread | | | Date | Mon, 5 Oct 2009 11:48:49 +0200 | From | Ingo Molnar <> | Subject | Re: [RFC PATCH] perf_core: provide a kernel-internal interface to get to performance counters |
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* 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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