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

Felipe Franciosi
Thu, 12 Nov 2015 12:02:09 -0000

Hi Jason,

I understand your busy loop timeout is quite conservative at 50us. Did you try
any other values?

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)?


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.

Thanks,
Felipe



On 12/11/2015 10:20, "kvm-owner at vger.kernel.org on behalf of Jason
Wang" <kvm-owner at vger.kernel.org on behalf of jasowang at
redhat.com> 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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