linux-kernel - Re: [PATCH v8 0/3]: perf: reduce data loss when profiling highly parallel CPU bound workloads

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Date:   Mon, 10 Sep 2018 12:23:28 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 0/3]: perf: reduce data loss when profiling highly
 parallel CPU bound workloads

On Mon, Sep 10, 2018 at 12:13:25PM +0200, Ingo Molnar wrote:
> 
> * Jiri Olsa <jolsa@...hat.com> wrote:
> 
> > On Mon, Sep 10, 2018 at 12:03:03PM +0200, Ingo Molnar wrote:
> > > 
> > > * Jiri Olsa <jolsa@...hat.com> wrote:
> > > 
> > > > > Per-CPU threading the record session would have so many other advantages as well (scalability, 
> > > > > etc.).
> > > > > 
> > > > > Jiri did per-CPU recording patches a couple of months ago, not sure how usable they are at the 
> > > > > moment?
> > > > 
> > > > it's still usable, I can rebase it and post a branch pointer,
> > > > the problem is I haven't been able to find a case with a real
> > > > performance benefit yet.. ;-)
> > > > 
> > > > perhaps because I haven't tried on server with really big cpu
> > > > numbers
> > > 
> > > Maybe Alexey could pick up from there? Your concept looked fairly mature to me
> > > and I tried it on a big-CPU box back then and there were real improvements.
> > 
> > too bad u did not share your results, it could have been already in ;-)
> 
> Yeah :-/ Had a proper round of testing on my TODO, then the big box I'd have tested it on
> broke ...
> 
> > let me rebase/repost once more and let's see
> 
> Thanks!
> 
> > I think we could benefit from both multiple threads event reading
> > and AIO writing for perf.data.. it could be merged together
> 
> So instead of AIO writing perf.data, why not just turn perf.data into a directory structure 
> with per CPU files? That would allow all sorts of neat future performance features such as 

that's basically what the multiple-thread record patchset does

jirka

> mmap() or splice() based zero-copy.
> 
> User-space post-processing can then read the files and put them into global order - or use the 
> per CPU nature of them, which would be pretty useful too.
> 
> Also note how well this works on NUMA as well, as the backing pages would be allocated in a 
> NUMA-local fashion.
> 
> I.e. the whole per-CPU threading would enable such a separation of the tracing/event streams 
> and would allow true scalability.
> 
> Thanks,
> 
> 	Ingo