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

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Message-ID: <74e64322-1f79-d072-cc0e-8d1d09d4ab89@linux.intel.com>
Date:   Tue, 11 Sep 2018 11:16:45 +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 11.09.2018 9:35, Ingo Molnar wrote:
> 
> * Alexey Budankov <alexey.budankov@...ux.intel.com> wrote:
> 
>> It may sound too optimistic but glibc API is expected to be backward compatible 
>> and for POSIX AIO API part too. Internal implementation also tends to evolve to 
>> better option overtime, more probably basing on modern kernel capabilities 
>> mentioned here: http://man7.org/linux/man-pages/man2/io_submit.2.html
> 
> I'm not talking about compatibility, and I'm not just talking about glibc, perf works under 
> other libcs as well - and let me phrase it in another way: basic event handling, threading, 
> scheduling internals should be a *core competency* of a tracing/profiling tool.

Well, the requirement of independence from some specific libc implementation 
as well as *core competency* design approach clarify a lot. Thanks!

> 
> I.e. we might end up using the exact same per event fd thread pool design that glibc uses 
> currently. Or not. Having that internal and open coded to perf, like Jiri has started 
> implementing it, allows people to experiment with it.

My point here is that following some standardized programming models and APIs 
(like POSIX) in the tool code, even if the tool itself provides internal open 
coded implementation for the APIs, would simplify experimenting with the tool 
as well as lower barriers for new comers. Perf project could benefit from that.

> 
> This isn't some GUI toolkit, this is at the essence of perf, and we are not very good on large 
> systems right now, and I think the design should be open-coded threading, not relying on an 
> (perf-)external AIO library to get it right.
> 
> The glibc thread pool implementation of POSIX AIO is basically a fall-back 
> implementation, for the case where there's no native KAIO interface to rely on.
> 
>> Well, explicit threading in the tool for AIO, in the simplest case, means 
>> incorporating some POSIX API implementation into the tool, avoiding 
>> code reuse in the first place. That tends to be error prone and costly.
> 
> It's a core competency, we better do it right and not outsource it.

Yep, makes sense.

Thanks!
Alexey

> 
> Please take a look at Jiri's patches (once he re-posts them), I think it's a very good 
> starting point.
> 
> Thanks,
> 
> 	Ingo
>