From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-391394-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 12182 invoked by alias); 9 May 2012 13:29:40 -0000
Received: (qmail 12173 invoked by uid 22791); 9 May 2012 13:29:39 -0000
X-SWARE-Spam-Status: No, hits=-4.3 required=5.0	tests=ALL_TRUSTED,AWL,BAYES_00,KHOP_THREADED
X-Spam-Check-By: sourceware.org
Received: from localhost (HELO gcc.gnu.org) (127.0.0.1)    by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Wed, 09 May 2012 13:29:21 +0000
From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/53292] multi-threaded (OpenMP) is slower than single-threaded
Date: Wed, 09 May 2012 13:33:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Keywords:
X-Bugzilla-Severity: normal
X-Bugzilla-Who: jakub at gcc dot gnu.org
X-Bugzilla-Status: RESOLVED
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Changed-Fields: Status Resolution
Message-ID: <bug-53292-4-MQZlAQEiQA@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-53292-4@http.gcc.gnu.org/bugzilla/>
References: <bug-53292-4@http.gcc.gnu.org/bugzilla/>
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
Content-Type: text/plain; charset="UTF-8"
MIME-Version: 1.0
Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-bugs.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-help@gcc.gnu.org>
Sender: gcc-bugs-owner@gcc.gnu.org
X-SW-Source: 2012-05/txt/msg01012.txt.bz2

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53292

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-05-09 13:29:00 UTC ---
Sorry, missed you aren't measuring it with the single-threaded loop.  Anyway,
the test is still not relevant, it is purely memory bound, and as you can see
from running it with very small arguments, the thread creation and omp for
initial overhead is in the noise, what you see is just how the cache hierarchy
of your CPU works.  The inner loop in which all the measured time is spent in
is very similar (and even if hand edited to be identical it doesn't help at
all).