From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758850AbcG1BqB (ORCPT ); Wed, 27 Jul 2016 21:46:01 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42136 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752142AbcG1Bp7 (ORCPT ); Wed, 27 Jul 2016 21:45:59 -0400 Date: Thu, 28 Jul 2016 04:45:52 +0300 From: "Michael S. Tsirkin" To: "Li, Liang Z" Cc: "Hansen, Dave" , "linux-kernel@vger.kernel.org" , "virtualization@lists.linux-foundation.org" , "linux-mm@kvack.org" , "virtio-dev@lists.oasis-open.org" , "kvm@vger.kernel.org" , "qemu-devel@nongnu.org" , "dgilbert@redhat.com" , "quintela@redhat.com" , Andrew Morton , Vlastimil Babka , Mel Gorman , Paolo Bonzini , Cornelia Huck , Amit Shah Subject: Re: [PATCH v2 repost 4/7] virtio-balloon: speed up inflate/deflate process Message-ID: <20160728044000-mutt-send-email-mst@kernel.org> References: <1469582616-5729-1-git-send-email-liang.z.li@intel.com> <1469582616-5729-5-git-send-email-liang.z.li@intel.com> <5798DB49.7030803@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Thu, 28 Jul 2016 01:45:59 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 28, 2016 at 01:13:35AM +0000, Li, Liang Z wrote: > > Subject: Re: [PATCH v2 repost 4/7] virtio-balloon: speed up inflate/deflate > > process > > > > On 07/26/2016 06:23 PM, Liang Li wrote: > > > + vb->pfn_limit = VIRTIO_BALLOON_PFNS_LIMIT; > > > + vb->pfn_limit = min(vb->pfn_limit, get_max_pfn()); > > > + vb->bmap_len = ALIGN(vb->pfn_limit, BITS_PER_LONG) / > > > + BITS_PER_BYTE + 2 * sizeof(unsigned long); > > > + hdr_len = sizeof(struct balloon_bmap_hdr); > > > + vb->bmap_hdr = kzalloc(hdr_len + vb->bmap_len, GFP_KERNEL); > > > > This ends up doing a 1MB kmalloc() right? That seems a _bit_ big. How big > > was the pfn buffer before? > > Yes, it is if the max pfn is more than 32GB. > The size of the pfn buffer use before is 256*4 = 1024 Bytes, it's too small, > and it's the main reason for bad performance. > Use the max 1MB kmalloc is a balance between performance and flexibility, > a large page bitmap covers the range of all the memory is no good for a system > with huge amount of memory. If the bitmap is too small, it means we have > to traverse a long list for many times, and it's bad for performance. > > Thanks! > Liang There are all your implementation decisions though. If guest memory is so fragmented that you only have order 0 4k pages, then allocating a huge 1M contigious chunk is very problematic in and of itself. Most people rarely migrate and do not care how fast that happens. Wasting a large chunk of memory (and it's zeroed for no good reason, so you actually request host memory for it) for everyone to speed it up when it does happen is not really an option. -- MST