From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50147) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gB1oY-0001Ac-3w for qemu-devel@nongnu.org; Fri, 12 Oct 2018 14:05:30 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gB1oP-00035C-N0 for qemu-devel@nongnu.org; Fri, 12 Oct 2018 14:05:26 -0400 Date: Fri, 12 Oct 2018 14:05:13 -0400 From: "Michael S. Tsirkin" Message-ID: <20181012140306-mutt-send-email-mst@kernel.org> References: <20181012032431.32693-1-david@gibson.dropbear.id.au> <20181012032431.32693-2-david@gibson.dropbear.id.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181012032431.32693-2-david@gibson.dropbear.id.au> Subject: Re: [Qemu-devel] [RFC 1/5] virtio-balloon: Remove unnecessary MADV_WILLNEED on deflate List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: David Gibson Cc: dhildenb@redhat.com, imammedo@redhat.com, ehabkost@redhat.com, pbonzini@redhat.com, qemu-devel@nongnu.org, qemu-ppc@nongnu.org On Fri, Oct 12, 2018 at 02:24:27PM +1100, David Gibson wrote: > When the balloon is inflated, we discard memory place in it using madvise() > with MADV_DONTNEED. And when we deflate it we use MADV_WILLNEED, which > sounds like it makes sense but is actually unnecessary. > > The misleadingly named MADV_DONTNEED just discards the memory in question, > it doesn't set any persistent state on it in-kernel; all that's necessary > to bring the memory back is to touch it. MADV_WILLNEED in contrast > specifically says that the memory will be used soon and faults it in. > > Memory that's being given back to the guest by deflating the balloon > *might* be used soon, but it equally could just sit around in the guest's > pools until it actually needs it. And, over the general timescale that > memory ballooning operates, it seems unlikely that one extra fault for the > guest will be a vast performance issue. Thinking about it, it might be for RT guests. So I suspect if you want to drop MADV_WILLNEED you need a flag telling qemu that's not the usecase. > So, simplify the balloon's operation by dropping the madvise() on deflate. > > Signed-off-by: David Gibson > --- > hw/virtio/virtio-balloon.c | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) > > diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c > index 1728e4f83a..6ec4bcf4e1 100644 > --- a/hw/virtio/virtio-balloon.c > +++ b/hw/virtio/virtio-balloon.c > @@ -35,9 +35,8 @@ > > static void balloon_page(void *addr, int deflate) > { > - if (!qemu_balloon_is_inhibited()) { > - qemu_madvise(addr, BALLOON_PAGE_SIZE, > - deflate ? QEMU_MADV_WILLNEED : QEMU_MADV_DONTNEED); > + if (!qemu_balloon_is_inhibited() && !deflate) { > + qemu_madvise(addr, BALLOON_PAGE_SIZE, QEMU_MADV_DONTNEED); > } > } > > -- > 2.17.1