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 14:06:43 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.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


* Alexey Budankov <alexey.budankov@...ux.intel.com> wrote:

> Hi Ingo,
> 
> On 10.09.2018 12:18, Ingo Molnar wrote:
> > 
> > * Alexey Budankov <alexey.budankov@...ux.intel.com> wrote:
> > 
> >>
> >> Currently in record mode the tool implements trace writing serially. 
> >> The algorithm loops over mapped per-cpu data buffers and stores 
> >> ready data chunks into a trace file using write() system call.
> >>
> >> At some circumstances the kernel may lack free space in a buffer 
> >> because the other buffer's half is not yet written to disk due to 
> >> some other buffer's data writing by the tool at the moment.
> >>
> >> Thus serial trace writing implementation may cause the kernel 
> >> to loose profiling data and that is what observed when profiling 
> >> highly parallel CPU bound workloads on machines with big number 
> >> of cores.
> > 
> > Yay! I saw this frequently on a 120-CPU box (hw is broken now).
> > 
> >> Data loss metrics is the ratio lost_time/elapsed_time where 
> >> lost_time is the sum of time intervals containing PERF_RECORD_LOST 
> >> records and elapsed_time is the elapsed application run time 
> >> under profiling.
> >>
> >> Applying asynchronous trace streaming thru Posix AIO API
> >> (http://man7.org/linux/man-pages/man7/aio.7.html) 
> >> lowers data loss metrics value providing 2x improvement -
> >> lowering 98% loss to almost 0%.
> > 
> > Hm, instead of AIO why don't we use explicit threads instead? I think Posix AIO will fall back 
> > to threads anyway when there's no kernel AIO support (which there probably isn't for perf 
> > events).
> 
> Explicit threading is surely an option but having more threads 
> in the tool that stream performance data is a considerable 
> design complication.
> 
> Luckily, glibc AIO implementation is already based on pthreads, 
> but having a writing thread for every distinct fd only.

My argument is, we don't want to rely on glibc's choices here. They might
use a different threading design in the future, or it might differ between
libc versions.

The basic flow of tracing/profiling data is something we should control explicitly,
via explicit threading.

BTW., the usecase I was primarily concentrating on was a simpler one: 'perf record -a', not 
inherited workflow tracing. For system-wide profiling the ideal tracing setup is clean per-CPU 
separation, i.e. per CPU event fds, per CPU threads that read and then write into separate 
per-CPU files.

Thanks,

	Ingo