linux-kernel - Re: [PATCH 47/48] perf record: Spread maps for --threads option

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Message-ID: <20180924142254.GB4640@kernel.org>
Date:   Mon, 24 Sep 2018 11:22:54 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Namhyung Kim <namhyung@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andi Kleen <andi@...stfloor.org>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>,
        kernel-team@....com
Subject: Re: [PATCH 47/48] perf record: Spread maps for --threads option

Em Sun, Sep 23, 2018 at 09:44:32PM +0200, Jiri Olsa escreveu:
> On Mon, Sep 17, 2018 at 08:40:48PM +0900, Namhyung Kim wrote:
> > On Thu, Sep 13, 2018 at 02:54:49PM +0200, Jiri Olsa wrote:
> > > Currently we assign all maps to main thread. Adding
> > > code that spreads maps for --threads option.
> > > 
> > > For --thread option we create as many threads as there
> > > are memory maps in evlist, which is the number of CPUs
> > > in the system or CPUs we monitor. Each thread gets a
> > > single data mmap to read.
> > > 
> > > In addition we have also same amount of tracking mmaps
> > > for auxiliary events which we don't create special thread
> > > for. Instead we assign the to the main thread, because
> > > there's not much traffic expected there.
> > > 
> > > The assignment is visible from --thread-stats output:
> > > 
> > >           pid      write       poll       skip  maps (size 20K)
> > >     1s   9770       144B          1          0   19K   19K   19K   18K   19K
> > >          9772         0B          1          0   18K
> > >          9773         0B          1          0   19K
> > >          9774         0B          1          0   19K
> > > 
> > > There are 5 maps for thread 9770 (1 data map and 4 auxiliary)
> > > and one data map for every other thread. Each thread writes
> > > data to the separate data file.
> > 
> > Hmm.. not sure it'll work well for large machines with 1000+ cpus.
> > What about giving each thread a data mmap and a tracking mmap?
> 
> well currently we store the tracking data in single file,
> thats why we need just one thread to write them down

I agree with Namhyung, with a slight difference: perhaps we should set
perf_event_attr.mmap on one of the events of the per-cpu mmap, that way
we don't need that dummy event, right?
 
> with the *_time API, we should be able to properly read the
> tracking data separately for each cpu

That may end up making the *_time API not needed (assuming the kernel
keeps the per-cpu mmap events in order, barring that, using the
ordered_events in batches, prior to consuming the events) and would help
with things like 'perf top' and 'perf trace', that want to consume
events right away.

- Arnaldo