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=-3.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 461C3C4727D for ; Thu, 24 Sep 2020 10:47:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EA6572396E for ; Thu, 24 Sep 2020 10:47:33 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=citrix.com header.i=@citrix.com header.b="iNq7w93b" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726615AbgIXKrd (ORCPT ); Thu, 24 Sep 2020 06:47:33 -0400 Received: from esa6.hc3370-68.iphmx.com ([216.71.155.175]:2314 "EHLO esa6.hc3370-68.iphmx.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726303AbgIXKrc (ORCPT ); Thu, 24 Sep 2020 06:47:32 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=citrix.com; s=securemail; t=1600944451; h=date:from:to:cc:subject:message-id:references: mime-version:content-transfer-encoding:in-reply-to; bh=L8xE/00A85vyhQuQ/6lgUNXwsSSmV95Xl/TUzk3RuH8=; b=iNq7w93bELpIkcnJgq9SOfXSICZq2z1cER9w8Hy/xVBLq8WfBhY073Vn 4Q/If64zQ4AKJ/RLcBZwi4dNnvs7h6k+gRAmIwK1B6+vfi2exy+z2m6LO f9p9R4GsYk3WVG+bP4xL/2vjvtMtI2S+ez0Xc7bnssLp7KPuJ3PiKJlg5 A=; Authentication-Results: esa6.hc3370-68.iphmx.com; dkim=none (message not signed) header.i=none IronPort-SDR: opxQhHwtNFjKFi2WaTku1pymlx+LjDFtbAFzcvc/PQkiHp2uYJ0KdGnHs+Nkz+k99b4l26y18v 2ZScdqi2Bhx77+V9dTrc6KhF+rQq4xUf4vgW+o3/31mphJBRc+MW1sNtW+Cs7MSUh6ePbrfxGE i854802MMt9WN30p7+1I2VLdqaEbxmZH07axiKBL7m/buMcYHQ0VKyNtF7/remPTjbdFDu433s 5aIpn5KtbkEv0erRKGCeeMh01bjqMM6bE5OlPUdJjfTMoh/mdp2CqkljKuBK/YSISXRCbo8JRS buY= X-SBRS: None X-MesageID: 27746696 X-Ironport-Server: esa6.hc3370-68.iphmx.com X-Remote-IP: 162.221.158.21 X-Policy: $RELAYED X-IronPort-AV: E=Sophos;i="5.77,297,1596513600"; d="scan'208";a="27746696" Date: Thu, 24 Sep 2020 12:47:20 +0200 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: SeongJae Park CC: Konrad Rzeszutek Wilk , SeongJae Park , , , , , , , Subject: Re: [PATCH] xen-blkback: add a parameter for disabling of persistent grants Message-ID: <20200924104720.GO19254@Air-de-Roger> References: <20200924101344.GN19254@Air-de-Roger> <20200924102714.28141-1-sjpark@amazon.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20200924102714.28141-1-sjpark@amazon.com> X-ClientProxiedBy: AMSPEX02CAS01.citrite.net (10.69.22.112) To FTLPEX02CL06.citrite.net (10.13.108.179) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 24, 2020 at 12:27:14PM +0200, SeongJae Park wrote: > On Thu, 24 Sep 2020 12:13:44 +0200 "Roger Pau Monné" wrote: > > > On Wed, Sep 23, 2020 at 04:09:30PM -0400, Konrad Rzeszutek Wilk wrote: > > > On Tue, Sep 22, 2020 at 09:01:25AM +0200, SeongJae Park wrote: > > > > From: SeongJae Park > > > > > > > > Persistent grants feature provides high scalability. On some small > > > > systems, however, it could incur data copy overhead[1] and thus it is > > > > required to be disabled. But, there is no option to disable it. For > > > > the reason, this commit adds a module parameter for disabling of the > > > > feature. > > > > > > Would it be better suited to have it per guest? > > > > I think having a per-backend policy that could be specified at the > > toolstack level would be nice, but I see that as a further > > improvement. > > Agreed. > > > > > Having a global backend domain policy of whether persistent grants are > > enabled or not seems desirable, and if someone wants even more fine > > grained control this change is AFAICT not incompatible with a > > per-backend option anyway. > > I think we could extend this design by receiving list of exceptional domains. > For example, if 'feature_persistent' is True and exceptions list has '123, > 456', domains of domid 123 and 456 will not use persistent grants, and vice > versa. I think that would be quite fragile IMO, I wouldn't recommend relying on domain IDs. What I would do instead is add a new attribute to xl-disk-configuration [0] that allows setting the persistent grants usage on a per-disk basis, and that should be passed to blkback in a xenstore node. > I could implement this, but... to be honest, I don't really understand the > needs of the fine-grained control. AFAIU, the problem is 'scalability' vs > 'data copy overhead'. So, only small systems would want to turn persistent > grants off. In such a small system, why would we need fine-grained control? > I'm worrying if I would implement and maintain a feature without real use case. > > For the reason, I'd like to suggest to keep this as is for now and expand it > with the 'exceptions list' idea or something better, if a real use case comes > out later. I agree. I'm happy to take patches to implement more fine grained control, but that shouldn't prevent us from having a global policy if that's useful to users. Roger. [0] https://xenbits.xen.org/docs/unstable/man/xl-disk-configuration.5.html