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 13:40:17 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Ingo Molnar <mingo@...nel.org>
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

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.

> 
> 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?

Tool threads may contend, and actually do, with application 
threads, under heavy load when all CPU cores are utilized,
and this may alter performance profile.

Thanks,
Alexey

> 
> Thanks,
> 
> 	Ingo
>