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Re: [Qemu-devel] [PATCH v14 3/5] virtio-balloon: VIRTIO_BALLOON_F_SG

From: Wei Wang
Subject: Re: [Qemu-devel] [PATCH v14 3/5] virtio-balloon: VIRTIO_BALLOON_F_SG
Date: Fri, 18 Aug 2017 15:39:27 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 08/18/2017 10:22 AM, Michael S. Tsirkin wrote:
+static void send_balloon_page_sg(struct virtio_balloon *vb,
+                                struct virtqueue *vq,
+                                void *addr,
+                                uint32_t size)
+       unsigned int len;
+       int ret;
+       do {
+               ret = add_one_sg(vq, addr, size);
+               virtqueue_kick(vq);
+               wait_event(vb->acked, virtqueue_get_buf(vq, &len));
+               /*
+                * It is uncommon to see the vq is full, because the sg is sent
+                * one by one and the device is able to handle it in time. But
+                * if that happens, we go back to retry after an entry gets
+                * released.
+                */
Why send one by one though? Why not batch some s/gs and wait for all
of them to be completed? If memory if fragmented, waiting every time is
worse than what we have now (VIRTIO_BALLOON_ARRAY_PFNS_MAX at a time).

OK, I'll do batching in some fashion.


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