Re: [PATCH net V2] vhost: net: switch to use data copy if pending DMAs exceed the limit

[Ronen Hod <rhod@redhat.com>]

On 03/13/2014 09:28 AM, Jason Wang wrote:
> On 03/10/2014 04:03 PM, Michael S. Tsirkin wrote:
>> On Fri, Mar 07, 2014 at 01:28:27PM +0800, Jason Wang wrote:
>>>> We used to stop the handling of tx when the number of pending DMAs
>>>> exceeds VHOST_MAX_PEND. This is used to reduce the memory occupation
>>>> of both host and guest. But it was too aggressive in some cases, since
>>>> any delay or blocking of a single packet may delay or block the guest
>>>> transmission. Consider the following setup:
>>>>
>>>>      +-----+        +-----+
>>>>      | VM1 |        | VM2 |
>>>>      +--+--+        +--+--+
>>>>         |              |
>>>>      +--+--+        +--+--+
>>>>      | tap0|        | tap1|
>>>>      +--+--+        +--+--+
>>>>         |              |
>>>>      pfifo_fast   htb(10Mbit/s)
>>>>         |              |
>>>>      +--+--------------+---+
>>>>      |     bridge          |
>>>>      +--+------------------+
>>>>         |
>>>>      pfifo_fast
>>>>         |
>>>>      +-----+
>>>>      | eth0|(100Mbit/s)
>>>>      +-----+
>>>>
>>>> - start two VMs and connect them to a bridge
>>>> - add an physical card (100Mbit/s) to that bridge
>>>> - setup htb on tap1 and limit its throughput to 10Mbit/s
>>>> - run two netperfs in the same time, one is from VM1 to VM2. Another is
>>>>    from VM1 to an external host through eth0.
>>>> - result shows that not only the VM1 to VM2 traffic were throttled but
>>>>    also the VM1 to external host through eth0 is also throttled somehow.
>>>>
>>>> This is because the delay added by htb may lead the delay the finish
>>>> of DMAs and cause the pending DMAs for tap0 exceeds the limit
>>>> (VHOST_MAX_PEND). In this case vhost stop handling tx request until
>>>> htb send some packets. The problem here is all of the packets
>>>> transmission were blocked even if it does not go to VM2.
>>>>
>>>> We can solve this issue by relaxing it a little bit: switching to use
>>>> data copy instead of stopping tx when the number of pending DMAs
>>>> exceed half of the vq size. This is safe because:
>>>>
>>>> - The number of pending DMAs were still limited (half of the vq size)
>>>> - The out of order completion during mode switch can make sure that
>>>>    most of the tx buffers were freed in time in guest.
>>>>
>>>> So even if about 50% packets were delayed in zero-copy case, vhost
>>>> could continue to do the transmission through data copy in this case.
>>>>
>>>> Test result:
>>>>
>>>> Before this patch:
>>>> VM1 to VM2 throughput is 9.3Mbit/s
>>>> VM1 to External throughput is 40Mbit/s
>>>> CPU utilization is 7%
>>>>
>>>> After this patch:
>>>> VM1 to VM2 throughput is 9.3Mbit/s
>>>> Vm1 to External throughput is 93Mbit/s
>>>> CPU utilization is 16%
>>>>
>>>> Completed performance test on 40gbe shows no obvious changes in both
>>>> throughput and cpu utilization with this patch.
>>>>
>>>> The patch only solve this issue when unlimited sndbuf. We still need a
>>>> solution for limited sndbuf.
>>>>
>>>> Cc: Michael S. Tsirkin <mst@redhat.com>
>>>> Cc: Qin Chuanyu <qinchuanyu@huawei.com>
>>>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>> I thought hard about this.
>> Here's what worries me: if there are still head of line
>> blocking issues lurking in the stack, they will still
>> hurt guests such as windows which rely on timely
>> completion of buffers, but it makes it
>> that much harder to reproduce the problems with
>> linux guests which don't.
>> And this will make even it harder to figure out
>> whether zero copy is actually active to diagnose
>> high cpu utilization cases.
> Yes.
>>
>> So I think this is a good trick, but let's make
>> this path conditional on a new debugging module parameter:
>> how about head_of_line_blocking with default off?
> Sure. But the head of line blocking was only partially solved in the
> patch since we only support in-order completion of zerocopy packets.
> Maybe we need consider switching to out of order completion even for
> zerocopy skbs?

Yan, Dima,

I remember that there is an issue with out-of-order packets and WHQL.

Ronen.

>> This way if we suspect packets are delayed forever
>> somewhere, we can enable that and see guest networking block.
>>
>> Additionally, I think we should add a way to count zero copy
>> and non zero copy packets.
>> I see two ways to implement this: add tracepoints in vhost-net
>> or add counters in tun accessible with ethtool.
>> This can be a patch on top and does not have to block
>> this one though.
>>
> Yes, I post a RFC about 2 years ago, see
> https://lkml.org/lkml/2012/4/9/478 which only traces generic vhost
> behaviours. I can refresh this and add some -net specific tracepoints.
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