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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no 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 DA31FC4338F for ; Fri, 6 Aug 2021 07:00:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AFE3A611C6 for ; Fri, 6 Aug 2021 07:00:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243490AbhHFHA5 (ORCPT ); Fri, 6 Aug 2021 03:00:57 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]:57400 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243487AbhHFHA4 (ORCPT ); Fri, 6 Aug 2021 03:00:56 -0400 Received: from imap1.suse-dmz.suse.de (imap1.suse-dmz.suse.de [192.168.254.73]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id AA257223B4; Fri, 6 Aug 2021 07:00:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1628233240; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=gYRfQFPpoeE3yf0K02MkpUYYoJO/tHcf+RG54xZmCHk=; b=BzoeWUbMxehopPuLIzfy40/SOYFdeAUqBAmbSxgz61X7nwM0VDHeDiBw9PljFMgjBI+6fd Bbk2M35SVDHwwyY1PS8wRvkfONiJPVtTQJlFC5vRayqmbKURt/VXARekFKu4HHjronaemJ Xc08QcCE32vY8Gh+smUMi+FBt28cSsE= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1628233240; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=gYRfQFPpoeE3yf0K02MkpUYYoJO/tHcf+RG54xZmCHk=; b=3srs4CvwJw+Zc+6Q66HHQT50/TI2aIBLvvXR/daUZCv8YnIFUecLP0A4xk7OK0FpamogSK s29jaQZXw0vFcfAg== Received: from imap1.suse-dmz.suse.de (imap1.suse-dmz.suse.de [192.168.254.73]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap1.suse-dmz.suse.de (Postfix) with ESMTPS id 9ACA9136D9; Fri, 6 Aug 2021 07:00:40 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap1.suse-dmz.suse.de with ESMTPSA id EIK6JRjeDGGbDAAAGKfGzw (envelope-from ); Fri, 06 Aug 2021 07:00:40 +0000 Date: Fri, 6 Aug 2021 09:00:40 +0200 From: Daniel Wagner To: Alison Chaiken Cc: linux-rt-users Subject: Re: hyperthreading and RT latency Message-ID: <20210806070040.ryvka6lnpn24ink5@beryllium> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-rt-users@vger.kernel.org Hi Alison, On Thu, Aug 05, 2021 at 01:16:17PM -0700, Alison Chaiken wrote: > The advice in the RT wiki > > https://rt.wiki.kernel.org/index.php/HOWTO:_Build_an_RT-application#Hyper_threading > > about configuring the kernel and building RT applications was written > in 2014, when we were on the 3.x series. That makes one wonder how > relevant some of it is for the 5.x series, especially since processors > in common use have changed some since then. Some of the advice, > notably about power management, obviously is timeless. > > In particular, Daniel Wagner added: > > https://rt.wiki.kernel.org/index.php/HOWTO:_Build_an_RT-application#Hyper_threading > > "Hyper threading and also out of order execution of CPUs introduces > 'random' latencies. As mentioned in power management, it is > recommended to disable these feature (if possible) or carefully > benchmark the performance." > > Is the advice still current? I don't think the situation has changed. Though, I don't run these test on modern hardware on regular basis. The last time I played with it on a bit more modern hardware it was still measurable by cyclictest with hackbench as workload. > Should we RT-users all still turn hyperthreading off? Obviously, it depends on your use case. If your application can tolerate the added noise by SMT, you don't have to disable it. > Given the security vulnerabilities associated > with hyperthreading, there are clearly some use cases where doing so > is indicated anyway. Again it depends on your use case. Daniel