From: Stefano Garzarella <sgarzare@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
Stefan Hajnoczi <stefanha@redhat.com>,
"David S. Miller" <davem@davemloft.net>,
virtualization@lists.linux-foundation.org,
Jason Wang <jasowang@redhat.com>,
kvm@vger.kernel.org
Subject: Re: [PATCH v4 1/5] vsock/virtio: limit the memory used per-socket
Date: Tue, 30 Jul 2019 11:35:39 +0200 [thread overview]
Message-ID: <20190730093539.dcksure3vrykir3g@steredhat> (raw)
In-Reply-To: <20190729143622-mutt-send-email-mst@kernel.org>
On Mon, Jul 29, 2019 at 03:10:15PM -0400, Michael S. Tsirkin wrote:
> On Mon, Jul 29, 2019 at 06:50:56PM +0200, Stefano Garzarella wrote:
> > On Mon, Jul 29, 2019 at 06:19:03PM +0200, Stefano Garzarella wrote:
> > > On Mon, Jul 29, 2019 at 11:49:02AM -0400, Michael S. Tsirkin wrote:
> > > > On Mon, Jul 29, 2019 at 05:36:56PM +0200, Stefano Garzarella wrote:
> > > > > On Mon, Jul 29, 2019 at 10:04:29AM -0400, Michael S. Tsirkin wrote:
> > > > > > On Wed, Jul 17, 2019 at 01:30:26PM +0200, Stefano Garzarella wrote:
> > > > > > > Since virtio-vsock was introduced, the buffers filled by the host
> > > > > > > and pushed to the guest using the vring, are directly queued in
> > > > > > > a per-socket list. These buffers are preallocated by the guest
> > > > > > > with a fixed size (4 KB).
> > > > > > >
> > > > > > > The maximum amount of memory used by each socket should be
> > > > > > > controlled by the credit mechanism.
> > > > > > > The default credit available per-socket is 256 KB, but if we use
> > > > > > > only 1 byte per packet, the guest can queue up to 262144 of 4 KB
> > > > > > > buffers, using up to 1 GB of memory per-socket. In addition, the
> > > > > > > guest will continue to fill the vring with new 4 KB free buffers
> > > > > > > to avoid starvation of other sockets.
> > > > > > >
> > > > > > > This patch mitigates this issue copying the payload of small
> > > > > > > packets (< 128 bytes) into the buffer of last packet queued, in
> > > > > > > order to avoid wasting memory.
> > > > > > >
> > > > > > > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> > > > > > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > > > > >
> > > > > > This is good enough for net-next, but for net I think we
> > > > > > should figure out how to address the issue completely.
> > > > > > Can we make the accounting precise? What happens to
> > > > > > performance if we do?
> > > > > >
> > > > >
> > > > > In order to do more precise accounting maybe we can use the buffer size,
> > > > > instead of payload size when we update the credit available.
> > > > > In this way, the credit available for each socket will reflect the memory
> > > > > actually used.
> > > > >
> > > > > I should check better, because I'm not sure what happen if the peer sees
> > > > > 1KB of space available, then it sends 1KB of payload (using a 4KB
> > > > > buffer).
> > > > >
> > > > > The other option is to copy each packet in a new buffer like I did in
> > > > > the v2 [2], but this forces us to make a copy for each packet that does
> > > > > not fill the entire buffer, perhaps too expensive.
> > > > >
> > > > > [2] https://patchwork.kernel.org/patch/10938741/
> > > > >
> > > > >
> > > > > Thanks,
> > > > > Stefano
> > > >
> > > > Interesting. You are right, and at some level the protocol forces copies.
> > > >
> > > > We could try to detect that the actual memory is getting close to
> > > > admin limits and force copies on queued packets after the fact.
> > > > Is that practical?
> > >
> > > Yes, I think it is doable!
> > > We can decrease the credit available with the buffer size queued, and
> > > when the buffer size of packet to queue is bigger than the credit
> > > available, we can copy it.
> > >
> > > >
> > > > And yes we can extend the credit accounting to include buffer size.
> > > > That's a protocol change but maybe it makes sense.
> > >
> > > Since we send to the other peer the credit available, maybe this
> > > change can be backwards compatible (I'll check better this).
> >
> > What I said was wrong.
> >
> > We send a counter (increased when the user consumes the packets) and the
> > "buf_alloc" (the max memory allowed) to the other peer.
> > It makes a difference between a local counter (increased when the
> > packets are sent) and the remote counter to calculate the credit available:
> >
> > u32 virtio_transport_get_credit(struct virtio_vsock_sock *vvs, u32 credit)
> > {
> > u32 ret;
> >
> > spin_lock_bh(&vvs->tx_lock);
> > ret = vvs->peer_buf_alloc - (vvs->tx_cnt - vvs->peer_fwd_cnt);
> > if (ret > credit)
> > ret = credit;
> > vvs->tx_cnt += ret;
> > spin_unlock_bh(&vvs->tx_lock);
> >
> > return ret;
> > }
> >
> > Maybe I can play with "buf_alloc" to take care of bytes queued but not
> > used.
> >
> > Thanks,
> > Stefano
>
> Right. And the idea behind it all was that if we send a credit
> to remote then we have space for it.
Yes.
> I think the basic idea was that if we have actual allocated
> memory and can copy data there, then we send the credit to
> remote.
>
> Of course that means an extra copy every packet.
> So as an optimization, it seems that we just assume
> that we will be able to allocate a new buffer.
Yes, we refill the virtqueue when half of the buffers were used.
>
> First this is not the best we can do. We can actually do
> allocate memory in the socket before sending credit.
In this case, IIUC we should allocate an entire buffer (4KB),
so we can reuse it if the packet is big.
> If packet is small then we copy it there.
> If packet is big then we queue the packet,
> take the buffer out of socket and add it to the virtqueue.
>
> Second question is what to do about medium sized packets.
> Packet is 1K but buffer is 4K, what do we do?
> And here I wonder - why don't we add the 3K buffer
> to the vq?
This would allow us to have an accurate credit account.
The problem here is the compatibility. Before this series virtio-vsock
and vhost-vsock modules had the RX buffer size hard-coded
(VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE = 4K). So, if we send a buffer smaller
of 4K, there might be issues.
Maybe it is the time to add add 'features' to virtio-vsock device.
Thanks,
Stefano
next prev parent reply other threads:[~2019-07-30 9:35 UTC|newest]
Thread overview: 69+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-17 11:30 [PATCH v4 0/5] vsock/virtio: optimizations to increase the throughput Stefano Garzarella
2019-07-17 11:30 ` [PATCH v4 1/5] vsock/virtio: limit the memory used per-socket Stefano Garzarella
2019-07-29 14:04 ` Michael S. Tsirkin
2019-07-29 15:36 ` Stefano Garzarella
2019-07-29 15:49 ` Michael S. Tsirkin
2019-07-29 16:19 ` Stefano Garzarella
2019-07-29 16:50 ` Stefano Garzarella
2019-07-29 19:10 ` Michael S. Tsirkin
2019-07-30 9:35 ` Stefano Garzarella [this message]
2019-07-30 20:42 ` Michael S. Tsirkin
2019-08-01 10:47 ` Stefano Garzarella
2019-08-01 13:21 ` Michael S. Tsirkin
2019-08-01 13:36 ` Stefano Garzarella
2019-09-01 8:26 ` Michael S. Tsirkin
2019-09-01 10:17 ` Michael S. Tsirkin
2019-09-02 9:57 ` Stefano Garzarella
2019-09-02 15:23 ` Michael S. Tsirkin
2019-09-03 4:39 ` Michael S. Tsirkin
2019-09-03 7:45 ` Stefano Garzarella
2019-09-03 7:52 ` Michael S. Tsirkin
2019-09-03 8:00 ` Stefano Garzarella
2019-07-29 16:01 ` Michael S. Tsirkin
2019-07-29 16:41 ` Stefano Garzarella
2019-07-29 19:33 ` Michael S. Tsirkin
2019-08-30 9:40 ` Stefano Garzarella
2019-09-01 6:56 ` Michael S. Tsirkin
2019-09-02 8:39 ` Stefan Hajnoczi
2019-09-02 8:55 ` Stefano Garzarella
2019-10-11 13:40 ` Stefano Garzarella
2019-10-11 14:11 ` Michael S. Tsirkin
2019-10-11 14:23 ` Stefano Garzarella
2019-10-14 8:17 ` Stefan Hajnoczi
2019-10-14 8:21 ` Jason Wang
2019-10-14 8:38 ` Stefano Garzarella
2019-07-17 11:30 ` [PATCH v4 2/5] vsock/virtio: reduce credit update messages Stefano Garzarella
2019-07-22 8:36 ` Stefan Hajnoczi
2019-09-03 4:38 ` Michael S. Tsirkin
2019-09-03 7:31 ` Stefano Garzarella
2019-09-03 7:38 ` Michael S. Tsirkin
2019-07-17 11:30 ` [PATCH v4 3/5] vsock/virtio: fix locking in virtio_transport_inc_tx_pkt() Stefano Garzarella
2019-07-17 14:51 ` Michael S. Tsirkin
2019-07-18 7:43 ` Stefano Garzarella
2019-07-22 8:53 ` Stefan Hajnoczi
2019-07-17 11:30 ` [PATCH v4 4/5] vhost/vsock: split packets to send using multiple buffers Stefano Garzarella
2019-07-17 14:54 ` Michael S. Tsirkin
2019-07-18 7:50 ` Stefano Garzarella
2019-07-18 8:13 ` Michael S. Tsirkin
2019-07-18 9:37 ` Stefano Garzarella
2019-07-18 11:35 ` Michael S. Tsirkin
2019-07-19 8:08 ` Stefano Garzarella
2019-07-19 8:21 ` Jason Wang
2019-07-19 8:39 ` Stefano Garzarella
2019-07-19 8:51 ` Jason Wang
2019-07-19 9:20 ` Stefano Garzarella
2019-07-22 9:06 ` Stefan Hajnoczi
2019-07-17 11:30 ` [PATCH v4 5/5] vsock/virtio: change the maximum packet size allowed Stefano Garzarella
2019-07-17 14:59 ` Michael S. Tsirkin
2019-07-18 7:52 ` Stefano Garzarella
2019-07-18 12:33 ` Michael S. Tsirkin
2019-07-19 8:29 ` Stefano Garzarella
2019-07-22 9:07 ` Stefan Hajnoczi
2019-07-22 9:08 ` [PATCH v4 0/5] vsock/virtio: optimizations to increase the throughput Stefan Hajnoczi
2019-07-22 9:14 ` Stefano Garzarella
2019-07-29 13:12 ` Stefan Hajnoczi
2019-07-29 13:59 ` Michael S. Tsirkin
2019-07-30 9:40 ` Stefano Garzarella
2019-07-30 10:03 ` Jason Wang
2019-07-30 15:38 ` Stefano Garzarella
2019-09-03 8:02 ` request for stable (was Re: [PATCH v4 0/5] vsock/virtio: optimizations to increase the throughput) Michael S. Tsirkin
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