From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:48196) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eZvNT-0003vC-8o for qemu-devel@nongnu.org; Fri, 12 Jan 2018 04:11:56 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eZvNP-0003Hi-6T for qemu-devel@nongnu.org; Fri, 12 Jan 2018 04:11:55 -0500 Received: from mga05.intel.com ([192.55.52.43]:37459) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1eZvNO-0003Ge-Ii for qemu-devel@nongnu.org; Fri, 12 Jan 2018 04:11:50 -0500 Message-ID: <5A587C61.2010204@intel.com> Date: Fri, 12 Jan 2018 17:14:09 +0800 From: Wei Wang MIME-Version: 1.0 References: <1515501687-7874-1-git-send-email-wei.w.wang@intel.com> <201801092342.FCH56215.LJHOMVFFFOOSQt@I-love.SAKURA.ne.jp> <5A55EA71.6020309@intel.com> <201801112006.EHD48461.LOtVFFSOJMOFHQ@I-love.SAKURA.ne.jp> In-Reply-To: <201801112006.EHD48461.LOtVFFSOJMOFHQ@I-love.SAKURA.ne.jp> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v21 2/5 RESEND] virtio-balloon: VIRTIO_BALLOON_F_SG List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Tetsuo Handa , mst@redhat.com Cc: virtio-dev@lists.oasis-open.org, linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, linux-mm@kvack.org, mhocko@kernel.org, akpm@linux-foundation.org, mawilcox@microsoft.com, david@redhat.com, cornelia.huck@de.ibm.com, mgorman@techsingularity.net, aarcange@redhat.com, amit.shah@redhat.com, pbonzini@redhat.com, willy@infradead.org, liliang.opensource@gmail.com, yang.zhang.wz@gmail.com, quan.xu0@gmail.com, nilal@redhat.com, riel@redhat.com On 01/11/2018 07:06 PM, Tetsuo Handa wrote: > Wei Wang wrote: >> Michael, could we merge patch 3-5 first? > No! I'm repeatedly asking you to propose only VIRTIO_BALLOON_F_SG changes. > Please don't ignore me. > > > > Patch 4 depends on patch 2. Thus, back to patch 2. There is not strict dependence per se. I plan to split the two features into 2 series, and post out 3-5 first, and the corresponding hypervisor code. After that's done, I'll get back to the discussion of patch 2. > Now, proceeding to patch 4. > > Your patch is trying to call add_one_sg() for multiple times based on > > ---------------------------------------- > + /* > + * This is expected to never fail: there is always at least 1 entry > + * available on the vq, because when the vq is full the worker thread > + * that adds the sg will be put into sleep until at least 1 entry is > + * available to use. > + */ This will be more clear in the new version which is not together with patch 2. > > Now, I suspect we need to add VIRTIO_BALLOON_F_FREE_PAGE_VQ flag. I want to see > the patch for the hypervisor side which makes use of VIRTIO_BALLOON_F_FREE_PAGE_VQ > flag because its usage becomes tricky. Between the guest kernel obtains snapshot of > free memory blocks and the hypervisor is told that some pages are currently free, > these pages can become in use. That is, I don't think > > The second feature enables the optimization of the 1st round memory > transfer - the hypervisor can skip the transfer of guest free pages in the > 1st round. > > is accurate. The hypervisor is allowed to mark pages which are told as "currently > unused" by the guest kernel as "write-protected" before starting the 1st round. > Then, the hypervisor performs copying all pages except write-protected pages as > the 1st round. Then, the 2nd and later rounds will be the same. That is, > VIRTIO_BALLOON_F_FREE_PAGE_VQ requires the hypervisor to do 0th round as > preparation. Thus, I want to see the patch for the hypervisor side. > > Now, what if all free pages in the guest kernel were reserved as ballooned pages? > There will be no free pages which VIRTIO_BALLOON_F_FREE_PAGE_VQ flag would help. > The hypervisor will have to copy all pages because all pages are either currently > in-use or currently in balloons. After ballooning to appropriate size, there will > be little free memory in the guest kernel, and the hypervisor already knows which > pages are in the balloon. Thus, the hypervisor can skip copying the content of > pages in the balloon, without using VIRTIO_BALLOON_F_FREE_PAGE_VQ flag. > > Then, why can't we do "inflate the balloon up to reasonable level (e.g. no need to > wait for reclaim and no need to deflate)" instead of "find all the free pages as of > specific moment" ? That is, code for VIRTIO_BALLOON_F_DEFLATE_ON_OOM could be reused > instead of VIRTIO_BALLOON_F_FREE_PAGE_VQ ? > I think you misunderstood the work, which seems not easy to explain everything from the beginning here. I wish to review patch 4 (I'll send out a new independent version) with Michael if possible. I'll discuss with you about patch 2 later. Thanks. Best, Wei