From owner-freebsd-arch@FreeBSD.ORG Wed Feb 25 08:29:31 2015 Return-Path: Delivered-To: freebsd-arch@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 64324B76 for ; Wed, 25 Feb 2015 08:29:31 +0000 (UTC) Received: from mailout.easymail.ca (mailout.easymail.ca [64.68.201.169]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id F391BE87 for ; Wed, 25 Feb 2015 08:29:30 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by mailout.easymail.ca (Postfix) with ESMTP id 46C98E087 for ; Wed, 25 Feb 2015 03:29:29 -0500 (EST) X-Virus-Scanned: Debian amavisd-new at mailout.easymail.ca X-Spam-Flag: NO X-Spam-Score: -3.866 X-Spam-Level: X-Spam-Status: No, score=-3.866 required=5 tests=[ALL_TRUSTED=-1.8, AWL=-0.159, BAYES_00=-2.599, DNS_FROM_AHBL_RHSBL=0.692] Received: from mailout.easymail.ca ([127.0.0.1]) by localhost (easymail-mailout.easydns.vpn [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id k4-shd69UYLd for ; Wed, 25 Feb 2015 03:29:26 -0500 (EST) Received: from bsddt1241.lv01.astrodoggroup.com (unknown [40.141.24.126]) (using TLSv1 with cipher DHE-RSA-AES128-SHA (128/128 bits)) (No client certificate requested) by mailout.easymail.ca (Postfix) with ESMTPSA id D93A7E089 for ; Wed, 25 Feb 2015 03:29:25 -0500 (EST) Message-ID: <54ED87E9.8030706@astrodoggroup.com> Date: Wed, 25 Feb 2015 00:29:29 -0800 From: Harrison Grundy User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0 MIME-Version: 1.0 To: freebsd-arch@freebsd.org Subject: Re: locks and kernel randomness... References: <20150224015721.GT74514@kib.kiev.ua> <54EBDC1C.3060007@astrodoggroup.com> <20150224024250.GV74514@kib.kiev.ua> <20150224174053.GG46794@funkthat.com> <54ECBD4B.6000007@freebsd.org> <20150224182507.GI46794@funkthat.com> <54ECEA43.2080008@freebsd.org> <20150224231921.GQ46794@funkthat.com> <20150225002301.GS46794@funkthat.com> <54ED80BD.1080603@freebsd.org> In-Reply-To: <54ED80BD.1080603@freebsd.org> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit X-BeenThere: freebsd-arch@freebsd.org X-Mailman-Version: 2.1.18-1 Precedence: list List-Id: Discussion related to FreeBSD architecture List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 25 Feb 2015 08:29:31 -0000 On 02/24/15 23:58, Alfred Perlstein wrote: > > On 2/24/15 7:23 PM, John-Mark Gurney wrote: >> K. Macy wrote this message on Tue, Feb 24, 2015 at 15:33 -0800: >>>> If someone does find a performance issue w/ my patch, I WILL >>>> work with them on a solution, but I will not work w/ people >>>> who make unfounded claims about the impact of this work... >>>> >>> ... The concerns may be exaggerated, but they aren't >>> unfounded. Not quite the same thing, but no one wants to spend >>> the >> Till someone shows me code in the kernel tree where this is even >> close to a performance problem, it is unfounded... I've asked, >> and no one has >> >>> cycles doing a SHA256 because it's "The Right Thing"(tm) when >>> their use case only requires a fletcher2. >> Depends upon what you're doing.. I haven't proposed changing >> ZFS's default to sha256, so stop w/ the false equivalences... >> >>> If it doesn't already exist, it might also be worth looking in >>> to a more scalable CSPRNG implementation not requiring locking >>> in the common case. For example, each core is seeded separately >>> periodically so that has a private pool that is protected by a >>> critical section. The private pool would be regularly refreshed >>> by cpu-local callout. Thus, a lock would only be acquired if >>> the local entropy were depleted. >> I'm not discussing this until you read and reply to my original >> email, since it's clear that my original email's contents has >> been ignored in this thread... >> > What is final proposal? More spinlocks? That is not a good idea. > > Doing a single buildworld is not enough. Ask netflix or someone > with a real load of 1000s of threads/processing to do testing for > you if you truly want to touch scheduler. sched_ule runs this code once every .5 to 1.5 seconds, depending on the value of random, so using a CSPRNG there wouldn't actually be noticeable. (We're talking about a few thousand cycles, when the existing implementation has to make a remote memory read/write numpackages-1/numpackages percent of the time, which costs tens of thousands of cycles. Switching to a per-CPU CSPRNG is actually faster in those cases.) That being said, I believe the plan is to remove random() from sched_ule entirely. It doesn't need it to perform the balancing, and we can just use the LCG from cpu_search, if get_cyclecount isn't viable. --- Harrison