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

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Message-ID: <20180910091841.GA4664@gmail.com>
Date:   Mon, 10 Sep 2018 11:18:41 +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:

> 
> 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).

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?

Thanks,

	Ingo