Re: [Xen-devel] ring buffer overflow

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Fri, 2011-09-30 at 15:44 +0100, David Xu wrote:
> Hi,
> 
> 2011/9/29 Ian Campbell <Ian.Campbell@xxxxxxxxxx>:
> > On Fri, 2011-09-30 at 05:18 +0100, David Xu wrote:
> >> Hi,
> >>
> >> Does anybody know whether the ring buffer between front end and back
> >> end will suffer from overflow? I just wonder if the ring buffer will
> >> be full and drop some packets when the Net I/O load is very heavy.
> >
> > In the case of networking whichever end is putting stuff on the ring
> > checks that there is enough room and will stop the queue when it cannot
> > transmit any more and restart when room becomes available.
> 
> You mean even there is not enough room in ring buffer, xen will * not
> drop the packets * and just delay the transmission.

It's not Xen but rather the kernel back and front ends which is involved
here. You can examine the hard_start_xmit functions in both netback and
netfront to determine for yourself whether or not packets can be dropped
and when.

>  I used httperf to
> measure the performance of web server running in a VM (The workload in
> this VM is mixed, so it can not benefit from boost mechanism. The net
> i/o suffers from relatively high latency which depends on the number
> of VMs in the system). I found that with the increase of request rate
> in client side, the connection rate will drop and the connection time
> will increase dramatically. And the retransmission appears when the
> request rate is over than a quantum. So I doubted that the http/tcp
> connection suffer from the packets drop when the ring buffer is full
> because of high request rate.
> 
> >
> >> BTW, If I want to change the size of i/o ring buffer, how should I do?
> >> I tried to reset the NET_TX_RING_SIZE and NET_RX_RING_SIZE in both
> >> front end and back end, but it seems doesn't work. Thanks.
> >
> > Currently the rings are limited to 1 page so if you want to increase the
> > size you would need to add multipage ring support to the network
> > protocol. There have been patches to do this for the blk protocol but I
> > do not recall any for the net protocol.
> 
> Yes, increasing the size is relatively hard. So I just want reduce the
> size of ring buffer to make sure my doubt described above. I directly
> set  NET_TX_RING_SIZE and  NET_RX_RING_SIZE to 128, but it doesn't seem to 
> work.

You need to make sure both ends of the connection agree on the ring
size.

I'm afraid this is not a very common thing to want to do so if you want
to persist with this approach I'm afraid you'll have to do some
debugging.

Ian.



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