Re: [PATCH net-next RFC V3 0/3] basic busy polling support for vhost_net

From: Jason Wang
Date: Fri Nov 13 2015 - 04:21:14 EST



On 11/12/2015 08:02 PM, Felipe Franciosi wrote:
> Hi Jason,
>
> I understand your busy loop timeout is quite conservative at 50us. Did you try any other values?

I've also tried 20us. And results shows 50us was better in:

- very small packet tx (e.g 64bytes at most 46% improvement)
- TCP_RR (at most 11% improvement)

But I will test bigger values. In fact, for net itself, we can be even
more aggressive: make vhost poll forever but I haven't tired this.

>
> Also, did you measure how polling affects many VMs talking to each other (e.g. 20 VMs on each host, perhaps with several vNICs each, transmitting to a corresponding VM/vNIC pair on another host)?

Not yet, in my todo list.

>
>
> On a complete separate experiment (busy waiting on storage I/O rings on Xen), I have observed that bigger timeouts gave bigger benefits. On the other hand, all cases that contended for CPU were badly hurt with any sort of polling.
>
> The cases that contended for CPU consisted of many VMs generating workload over very fast I/O devices (in that case, several NVMe devices on a single host). And the metric that got affected was aggregate throughput from all VMs.
>
> The solution was to determine whether to poll depending on the host's overall CPU utilisation at that moment. That gave me the best of both worlds as polling made everything faster without slowing down any other metric.

You mean a threshold and exit polling when it exceeds this? I use a
simpler method: just exit the busy loop when there's more than one
processes is in running state. I test this method in the past for socket
busy read (http://www.gossamer-threads.com/lists/linux/kernel/1997531)
which seems can solve the issue. But haven't tested this for vhost
polling. Will run some simple test (e.g pin two vhost threads in one
host cpu), and see how well it perform.

Thanks

>
> Thanks,
> Felipe
>
>
>
> On 12/11/2015 10:20, "kvm-owner@xxxxxxxxxxxxxxx on behalf of Jason Wang" <kvm-owner@xxxxxxxxxxxxxxx on behalf of jasowang@xxxxxxxxxx> wrote:
>
>>
>> On 11/12/2015 06:16 PM, Jason Wang wrote:
>>> Hi all:
>>>
>>> This series tries to add basic busy polling for vhost net. The idea is
>>> simple: at the end of tx/rx processing, busy polling for new tx added
>>> descriptor and rx receive socket for a while. The maximum number of
>>> time (in us) could be spent on busy polling was specified ioctl.
>>>
>>> Test were done through:
>>>
>>> - 50 us as busy loop timeout
>>> - Netperf 2.6
>>> - Two machines with back to back connected ixgbe
>>> - Guest with 1 vcpu and 1 queue
>>>
>>> Results:
>>> - For stream workload, ioexits were reduced dramatically in medium
>>> size (1024-2048) of tx (at most -39%) and almost all rx (at most
>>> -79%) as a result of polling. This compensate for the possible
>>> wasted cpu cycles more or less. That porbably why we can still see
>>> some increasing in the normalized throughput in some cases.
>>> - Throughput of tx were increased (at most 105%) expect for the huge
>>> write (16384). And we can send more packets in the case (+tpkts were
>>> increased).
>>> - Very minor rx regression in some cases.
>>> - Improvemnt on TCP_RR (at most 16%).
>> Forget to mention, the following test results by order are:
>>
>> 1) Guest TX
>> 2) Guest RX
>> 3) TCP_RR
>>
>>> size/session/+thu%/+normalize%/+tpkts%/+rpkts%/+ioexits%/
>>> 64/ 1/ +9%/ -17%/ +5%/ +10%/ -2%
>>> 64/ 2/ +8%/ -18%/ +6%/ +10%/ -1%
>>> 64/ 4/ +4%/ -21%/ +6%/ +10%/ -1%
>>> 64/ 8/ +9%/ -17%/ +6%/ +9%/ -2%
>>> 256/ 1/ +20%/ -1%/ +15%/ +11%/ -9%
>>> 256/ 2/ +15%/ -6%/ +15%/ +8%/ -8%
>>> 256/ 4/ +17%/ -4%/ +16%/ +8%/ -8%
>>> 256/ 8/ -61%/ -69%/ +16%/ +10%/ -10%
>>> 512/ 1/ +15%/ -3%/ +19%/ +18%/ -11%
>>> 512/ 2/ +19%/ 0%/ +19%/ +13%/ -10%
>>> 512/ 4/ +18%/ -2%/ +18%/ +15%/ -10%
>>> 512/ 8/ +17%/ -1%/ +18%/ +15%/ -11%
>>> 1024/ 1/ +25%/ +4%/ +27%/ +16%/ -21%
>>> 1024/ 2/ +28%/ +8%/ +25%/ +15%/ -22%
>>> 1024/ 4/ +25%/ +5%/ +25%/ +14%/ -21%
>>> 1024/ 8/ +27%/ +7%/ +25%/ +16%/ -21%
>>> 2048/ 1/ +32%/ +12%/ +31%/ +22%/ -38%
>>> 2048/ 2/ +33%/ +12%/ +30%/ +23%/ -36%
>>> 2048/ 4/ +31%/ +10%/ +31%/ +24%/ -37%
>>> 2048/ 8/ +105%/ +75%/ +33%/ +23%/ -39%
>>> 16384/ 1/ 0%/ -14%/ +2%/ 0%/ +19%
>>> 16384/ 2/ 0%/ -13%/ +19%/ -13%/ +17%
>>> 16384/ 4/ 0%/ -12%/ +3%/ 0%/ +2%
>>> 16384/ 8/ 0%/ -11%/ -2%/ +1%/ +1%
>>> size/session/+thu%/+normalize%/+tpkts%/+rpkts%/+ioexits%/
>>> 64/ 1/ -7%/ -23%/ +4%/ +6%/ -74%
>>> 64/ 2/ -2%/ -12%/ +2%/ +2%/ -55%
>>> 64/ 4/ +2%/ -5%/ +10%/ -2%/ -43%
>>> 64/ 8/ -5%/ -5%/ +11%/ -34%/ -59%
>>> 256/ 1/ -6%/ -16%/ +9%/ +11%/ -60%
>>> 256/ 2/ +3%/ -4%/ +6%/ -3%/ -28%
>>> 256/ 4/ 0%/ -5%/ -9%/ -9%/ -10%
>>> 256/ 8/ -3%/ -6%/ -12%/ -9%/ -40%
>>> 512/ 1/ -4%/ -17%/ -10%/ +21%/ -34%
>>> 512/ 2/ 0%/ -9%/ -14%/ -3%/ -30%
>>> 512/ 4/ 0%/ -4%/ -18%/ -12%/ -4%
>>> 512/ 8/ -1%/ -4%/ -1%/ -5%/ +4%
>>> 1024/ 1/ 0%/ -16%/ +12%/ +11%/ -10%
>>> 1024/ 2/ 0%/ -11%/ 0%/ +5%/ -31%
>>> 1024/ 4/ 0%/ -4%/ -7%/ +1%/ -22%
>>> 1024/ 8/ -5%/ -6%/ -17%/ -29%/ -79%
>>> 2048/ 1/ 0%/ -16%/ +1%/ +9%/ -10%
>>> 2048/ 2/ 0%/ -12%/ +7%/ +9%/ -26%
>>> 2048/ 4/ 0%/ -7%/ -4%/ +3%/ -64%
>>> 2048/ 8/ -1%/ -5%/ -6%/ +4%/ -20%
>>> 16384/ 1/ 0%/ -12%/ +11%/ +7%/ -20%
>>> 16384/ 2/ 0%/ -7%/ +1%/ +5%/ -26%
>>> 16384/ 4/ 0%/ -5%/ +12%/ +22%/ -23%
>>> 16384/ 8/ 0%/ -1%/ -8%/ +5%/ -3%
>>> size/session/+thu%/+normalize%/+tpkts%/+rpkts%/+ioexits%/
>>> 1/ 1/ +9%/ -29%/ +9%/ +9%/ +9%
>>> 1/ 25/ +6%/ -18%/ +6%/ +6%/ -1%
>>> 1/ 50/ +6%/ -19%/ +5%/ +5%/ -2%
>>> 1/ 100/ +5%/ -19%/ +4%/ +4%/ -3%
>>> 64/ 1/ +10%/ -28%/ +10%/ +10%/ +10%
>>> 64/ 25/ +8%/ -18%/ +7%/ +7%/ -2%
>>> 64/ 50/ +8%/ -17%/ +8%/ +8%/ -1%
>>> 64/ 100/ +8%/ -17%/ +8%/ +8%/ -1%
>>> 256/ 1/ +10%/ -28%/ +10%/ +10%/ +10%
>>> 256/ 25/ +15%/ -13%/ +15%/ +15%/ 0%
>>> 256/ 50/ +16%/ -14%/ +18%/ +18%/ +2%
>>> 256/ 100/ +15%/ -13%/ +12%/ +12%/ -2%
>>>
>>> Changes from V2:
>>> - poll also at the end of rx handling
>>> - factor out the polling logic and optimize the code a little bit
>>> - add two ioctls to get and set the busy poll timeout
>>> - test on ixgbe (which can give more stable and reproducable numbers)
>>> instead of mlx4.
>>>
>>> Changes from V1:
>>> - Add a comment for vhost_has_work() to explain why it could be
>>> lockless
>>> - Add param description for busyloop_timeout
>>> - Split out the busy polling logic into a new helper
>>> - Check and exit the loop when there's a pending signal
>>> - Disable preemption during busy looping to make sure lock_clock() was
>>> correctly used.
>>>
>>> Jason Wang (3):
>>> vhost: introduce vhost_has_work()
>>> vhost: introduce vhost_vq_more_avail()
>>> vhost_net: basic polling support
>>>
>>> drivers/vhost/net.c | 77 +++++++++++++++++++++++++++++++++++++++++++---
>>> drivers/vhost/vhost.c | 48 +++++++++++++++++++++++------
>>> drivers/vhost/vhost.h | 3 ++
>>> include/uapi/linux/vhost.h | 11 +++++++
>>> 4 files changed, 125 insertions(+), 14 deletions(-)
>>>
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