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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 1A0AEC169C4 for ; Mon, 11 Feb 2019 19:56:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E5345218AD for ; Mon, 11 Feb 2019 19:56:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387875AbfBKT4n (ORCPT ); Mon, 11 Feb 2019 14:56:43 -0500 Received: from mx1.redhat.com ([209.132.183.28]:57742 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728293AbfBKT4m (ORCPT ); Mon, 11 Feb 2019 14:56:42 -0500 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id BBDB980F81; Mon, 11 Feb 2019 19:56:41 +0000 (UTC) Received: from redhat.com (ovpn-120-40.rdu2.redhat.com [10.10.120.40]) by smtp.corp.redhat.com (Postfix) with SMTP id 04D2C6091C; Mon, 11 Feb 2019 19:56:39 +0000 (UTC) Date: Mon, 11 Feb 2019 14:56:39 -0500 From: "Michael S. Tsirkin" To: Dave Hansen Cc: Alexander Duyck , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, rkrcmar@redhat.com, alexander.h.duyck@linux.intel.com, x86@kernel.org, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, pbonzini@redhat.com, tglx@linutronix.de, akpm@linux-foundation.org Subject: Re: [RFC PATCH 3/4] kvm: Add guest side support for free memory hints Message-ID: <20190211145531-mutt-send-email-mst@kernel.org> References: <20190204181118.12095.38300.stgit@localhost.localdomain> <20190204181552.12095.46287.stgit@localhost.localdomain> <20190209194437-mutt-send-email-mst@kernel.org> <0d12ccec-d05f-80b8-9498-710d521c81d2@intel.com> <20190211124925-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Mon, 11 Feb 2019 19:56:42 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 11, 2019 at 10:19:17AM -0800, Dave Hansen wrote: > On 2/11/19 9:58 AM, Michael S. Tsirkin wrote: > >>> Really it seems we want a virtio ring so we can pass a batch of these. > >>> E.g. 256 entries, 2M each - that's more like it. > >> That only makes sense for a system that's doing high-frequency, > >> discontiguous frees of 2M pages. Right now, a 2M free/realloc cycle > >> (THP or hugetlb) is *not* super-high frequency just because of the > >> latency for zeroing the page. > > Heh but with a ton of free memory, and a thread zeroing some of > > it out in the background, will this still be the case? > > It could be that we'll be able to find clean pages > > at all times. > > In a systems where we have some asynchrounous zeroing of memory where > freed, non-zeroed memory is sequestered out of the allocator, yeah, that > could make sense. > > But, that's not what we have today. Right. I wonder whether it's smart to build this assumption into a host/guest interface though. > >> A virtio ring seems like an overblown solution to a non-existent problem. > > It would be nice to see some traces to help us decide one way or the other. > > Yeah, agreed. Sounds like we need some more testing to see if these > approaches hit bottlenecks anywhere.