Messages in this thread | | | Date | Mon, 5 Oct 2009 12:08:55 +0200 | Subject | Re: [RFC PATCH] perf_core: provide a kernel-internal interface to get to performance counters | From | Frédéric Weisbecker <> |
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Le 5 octobre 2009 11:48, Ingo Molnar <mingo@elte.hu> a écrit : > > * 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 >
Ok. The third solution solves the multi-channel problem, and for the ordering...well as you said, everything has a cost. -- 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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