From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:45973) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gu8p5-0002EK-6a for qemu-devel@nongnu.org; Wed, 13 Feb 2019 23:40:32 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gu8on-0006up-L7 for qemu-devel@nongnu.org; Wed, 13 Feb 2019 23:40:18 -0500 From: David Gibson Date: Thu, 14 Feb 2019 15:39:12 +1100 Message-Id: <20190214043916.22128-2-david@gibson.dropbear.id.au> In-Reply-To: <20190214043916.22128-1-david@gibson.dropbear.id.au> References: <20190214043916.22128-1-david@gibson.dropbear.id.au> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Subject: [Qemu-devel] [PATCH 1/5] virtio-balloon: Remove unnecessary MADV_WILLNEED on deflate List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: mst@redhat.com, qemu-devel@nongnu.org Cc: qemu-ppc@nongnu.org, David Gibson , David Hildenbrand 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. This patch simplify's the balloon operation by dropping the madvise() on deflate. This might have an impact on performance - it will move a delay at deflate time until that memory is actually touched, which might be more latency sensitive. However: * 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 needed (or even be faulted out again if the host is under memory pressure). * Usually, the timescale over which you'll be adjusting the balloon is long enough that a few extra faults after deflation aren't going to make a difference. Signed-off-by: David Gibson Reviewed-by: David Hildenbrand Reviewed-by: Michael S. Tsirkin --- 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 a12677d4d5..43af521884 100644 --- a/hw/virtio/virtio-balloon.c +++ b/hw/virtio/virtio-balloon.c @@ -35,9 +35,8 @@ =20 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); } } =20 --=20 2.20.1