From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrea Arcangeli Subject: Re: [Qemu-devel] [PATCH kernel v5 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration Date: Wed, 7 Dec 2016 19:38:17 +0100 Message-ID: <20161207183817.GE28786__5904.88189268417$1481136798$gmane$org@redhat.com> References: <1480495397-23225-1-git-send-email-liang.z.li@intel.com> <0b18c636-ee67-cbb4-1ba3-81a06150db76@redhat.com> <0b83db29-ebad-2a70-8d61-756d33e33a48@intel.com> <2171e091-46ee-decd-7348-772555d3a5e3@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Dave Hansen Cc: "mhocko@suse.com" , "kvm@vger.kernel.org" , "mst@redhat.com" , "Li, Liang Z" , "linux-kernel@vger.kernel.org" , "qemu-devel@nongnu.org" , "linux-mm@kvack.org" , "kirill.shutemov@linux.intel.com" , "pbonzini@redhat.com" , "akpm@linux-foundation.org" , "virtualization@lists.linux-foundation.org" , "dgilbert@redhat.com" List-Id: virtualization@lists.linuxfoundation.org Hello, On Wed, Dec 07, 2016 at 08:57:01AM -0800, Dave Hansen wrote: > It is more space-efficient. We're fitting the order into 6 bits, which > would allows the full 2^64 address space to be represented in one entry, Very large order is the same as very large len, 6 bits of order or 8 bytes of len won't really move the needle here, simpler code is preferable. The main benefit of "len" is that it can be more granular, plus it's simpler than the bitmap too. Eventually all this stuff has to end up into a madvisev (not yet upstream but somebody posted it for jemalloc and should get merged eventually). So the bitmap shall be demuxed to a addr,len array anyway, the bitmap won't ever be sent to the madvise syscall, which makes the intermediate representation with the bitmap a complication with basically no benefits compared to a (N, [addr1,len1], .., [addrN, lenN]) representation. If you prefer 1 byte of order (not just 6 bits) instead 8bytes of len that's possible too, I wouldn't be against that, the conversion before calling madvise would be pretty efficient too. > and leaves room for the bitmap size to be encoded as well, if we decide > we need a bitmap in the future. How would a bitmap ever be useful with very large page-order? > If that was purely a length, we'd be limited to 64*4k pages per entry, > which isn't even a full large page. I don't follow here. What we suggest is to send the data down represented as (N, [addr1,len1], ..., [addrN, lenN]) which allows infinite ranges each one of maximum length 2^64, so 2^64 multiplied infinite times if you wish. Simplifying the code and not having any bitmap at all and no :6 :6 bits either. The high order to low order loop of allocations is the interesting part here, not the bitmap, and the fact of doing a single vmexit to send the large ranges. Once we pull out the largest order regions, we just add them to the array as [addr,1UL<