Re: [Xen-devel] ring buffer overflow

[David Xu <davidxu06@xxxxxxxxx>]

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. 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.

Regards,
Cong

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