From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 90D20C433F5 for ; Mon, 10 Sep 2018 09:59:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4327F2086E for ; Mon, 10 Sep 2018 09:59:14 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4327F2086E Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728168AbeIJOw1 (ORCPT ); Mon, 10 Sep 2018 10:52:27 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:34644 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726738AbeIJOw1 (ORCPT ); Mon, 10 Sep 2018 10:52:27 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 36B7C4023ECC; Mon, 10 Sep 2018 09:59:11 +0000 (UTC) Received: from krava (unknown [10.43.17.10]) by smtp.corp.redhat.com (Postfix) with SMTP id D1CDC20229CA; Mon, 10 Sep 2018 09:59:09 +0000 (UTC) Date: Mon, 10 Sep 2018 11:59:09 +0200 From: Jiri Olsa To: Ingo Molnar Cc: Alexey Budankov , Peter Zijlstra , Arnaldo Carvalho de Melo , Alexander Shishkin , Namhyung Kim , Andi Kleen , linux-kernel Subject: Re: [PATCH v8 0/3]: perf: reduce data loss when profiling highly parallel CPU bound workloads Message-ID: <20180910095909.GA15548@krava> References: <20180910091841.GA4664@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180910091841.GA4664@gmail.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.78 on 10.11.54.4 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.7]); Mon, 10 Sep 2018 09:59:11 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.7]); Mon, 10 Sep 2018 09:59:11 +0000 (UTC) for IP:'10.11.54.4' DOMAIN:'int-mx04.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'jolsa@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 10, 2018 at 11:18:41AM +0200, Ingo Molnar wrote: > > * Alexey Budankov 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). this patch adds the aoi for writing to the perf.data file, reading of events is unchanged > > 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 jirka