From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:36185) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1S2MvK-0003rk-6t for qemu-devel@nongnu.org; Tue, 28 Feb 2012 08:17:03 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1S2Mv0-0006T1-7c for qemu-devel@nongnu.org; Tue, 28 Feb 2012 08:16:57 -0500 Received: from mx1.redhat.com ([209.132.183.28]:58084) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1S2Muz-0006So-Vx for qemu-devel@nongnu.org; Tue, 28 Feb 2012 08:16:38 -0500 Message-ID: <4F4CD3AF.7090709@redhat.com> Date: Tue, 28 Feb 2012 15:16:31 +0200 From: Avi Kivity MIME-Version: 1.0 References: <1333613dbb15f2b736394d77e795223e.squirrel@ssl.dlh.net> <2c2e4d6e-53f0-4698-8ad0-f4708b7987c6@email.android.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] linux guests and ksm performance List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: "peter.lieven@gmail.com" , qemu-devel@nongnu.org, kvm@vger.kernel.org On 02/24/2012 08:41 AM, Stefan Hajnoczi wrote: > > > > I dont think that it is cpu intense. All user pages are zeroed anyway, but at allocation time it shouldnt be a big difference in terms of cpu power. > > It's easy to find a scenario where eagerly zeroing pages is wasteful. > Imagine a process that uses all of physical memory. Once it > terminates the system is going to run processes that only use a small > set of pages. It's pointless zeroing all those pages if we're not > going to use them anymore. In the long term, we will use them, except if the guest is completely idle. The scenario in which zeroing is expensive is when the page is refilled through DMA. In that case the zeroing was wasted. This is a pretty common scenario in pagecache intensive workloads. -- error compiling committee.c: too many arguments to function