Date:	Wed, 9 Mar 2016 20:26:45 +0100
From:	Greg Kurz <gkurz@...ux.vnet.ibm.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	mst@...hat.com, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, yang.zhang.wz@...il.com,
	borntraeger@...ibm.com, RAPOPORT@...ibm.com
Subject: Re: [PATCH V4 0/3] basic busy polling support for vhost_net

On Fri,  4 Mar 2016 06:24:50 -0500
Jason Wang <jasowang@...hat.com> wrote:

> 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 A were done through:
> 
> - 50 us as busy loop timeout
> - Netperf 2.6
> - Two machines with back to back connected mlx4

Hi Jason,

Could this also improve performance if both VMs are
on the same host system ?

> - Guest with 1 vcpus and 1 queue
> 
> Results:
> - Obvious improvements (%5 - 20%) for latency (TCP_RR).
> - Get a better or minor regression on most of the TX tests, but see
>   some regression on 4096 size.
> - Except for 8 sessions of 4096 size RX, have a better or same
>   performance.
> - CPU utilization were incrased as expected.
> 
> TCP_RR:
> size/session/+thu%/+normalize%/+tpkts%/+rpkts%/+ioexits%/
>     1/     1/   +8%/  -32%/   +8%/   +8%/   +7%
>     1/    50/   +7%/  -19%/   +7%/   +7%/   +1%
>     1/   100/   +5%/  -21%/   +5%/   +5%/    0%
>     1/   200/   +5%/  -21%/   +7%/   +7%/   +1%
>    64/     1/  +11%/  -29%/  +11%/  +11%/  +10%
>    64/    50/   +7%/  -19%/   +8%/   +8%/   +2%
>    64/   100/   +8%/  -18%/   +9%/   +9%/   +2%
>    64/   200/   +6%/  -19%/   +6%/   +6%/    0%
>   256/     1/   +7%/  -33%/   +7%/   +7%/   +6%
>   256/    50/   +7%/  -18%/   +7%/   +7%/    0%
>   256/   100/   +9%/  -18%/   +8%/   +8%/   +2%
>   256/   200/   +9%/  -18%/  +10%/  +10%/   +3%
>  1024/     1/  +20%/  -28%/  +20%/  +20%/  +19%
>  1024/    50/   +8%/  -18%/   +9%/   +9%/   +2%
>  1024/   100/   +6%/  -19%/   +5%/   +5%/    0%
>  1024/   200/   +8%/  -18%/   +9%/   +9%/   +2%
> Guest TX:
> size/session/+thu%/+normalize%/+tpkts%/+rpkts%/+ioexits%/
>    64/     1/   -5%/  -28%/  +11%/  +12%/  +10%
>    64/     4/   -2%/  -26%/  +13%/  +13%/  +13%
>    64/     8/   -6%/  -29%/   +9%/  +10%/  +10%
>   512/     1/  +15%/   -7%/  +13%/  +11%/   +3%
>   512/     4/  +17%/   -6%/  +18%/  +13%/  +11%
>   512/     8/  +14%/   -7%/  +13%/   +7%/   +7%
>  1024/     1/  +27%/   -2%/  +26%/  +29%/  +12%
>  1024/     4/   +8%/   -9%/   +6%/   +1%/   +6%
>  1024/     8/  +41%/  +12%/  +34%/  +20%/   -3%
>  4096/     1/  -22%/  -21%/  -36%/  +81%/+1360%
>  4096/     4/  -57%/  -58%/ +286%/  +15%/+2074%
>  4096/     8/  +67%/  +70%/  -45%/   -8%/  +63%
> 16384/     1/   -2%/   -5%/   +5%/   -3%/  +80%
> 16384/     4/    0%/    0%/    0%/   +4%/ +138%
> 16384/     8/    0%/    0%/    0%/   +1%/  +41%
> 65535/     1/   -3%/   -6%/   +2%/  +11%/ +113%
> 65535/     4/   -2%/   -1%/   -2%/   -3%/ +484%
> 65535/     8/    0%/   +1%/    0%/   +2%/  +40%
> Guest RX:
> size/session/+thu%/+normalize%/+tpkts%/+rpkts%/+ioexits%/
>    64/     1/  +31%/   -3%/   +8%/   +8%/   +8%
>    64/     4/  +11%/  -17%/  +13%/  +14%/  +15%
>    64/     8/   +4%/  -23%/  +11%/  +11%/  +12%
>   512/     1/  +24%/    0%/  +18%/  +14%/   -8%
>   512/     4/   +4%/  -15%/   +6%/   +5%/   +6%
>   512/     8/  +26%/    0%/  +21%/  +10%/   +3%
>  1024/     1/  +88%/  +47%/  +69%/  +44%/  -30%
>  1024/     4/  +18%/   -5%/  +19%/  +16%/   +2%
>  1024/     8/  +15%/   -4%/  +13%/   +8%/   +1%
>  4096/     1/   -3%/   -5%/   +2%/   -2%/  +41%
>  4096/     4/   +2%/   +3%/  -20%/  -14%/  -24%
>  4096/     8/  -43%/  -45%/  +69%/  -24%/  +94%
> 16384/     1/   -3%/  -11%/  +23%/   +7%/  +42%
> 16384/     4/   -3%/   -3%/   -4%/   +5%/ +115%
> 16384/     8/   -1%/    0%/   -1%/   -3%/  +32%
> 65535/     1/   +1%/    0%/   +2%/    0%/  +66%
> 65535/     4/   -1%/   -1%/    0%/   +4%/ +492%
> 65535/     8/    0%/   -1%/   -1%/   +4%/  +38%
> 
> Changes from V3:
> - drop single_task_running()
> - use cpu_relax_lowlatency() instead of cpu_relax()
> 
> Changes from V2:
> - rename vhost_vq_more_avail() to vhost_vq_avail_empty(). And return
> false we __get_user() fails.
> - do not bother premmptions/timers for good path.
> - use vhost_vring_state as ioctl parameter instead of reinveting a new
> one.
> - add the unit of timeout (us) to the comment of new added ioctls
> 
> Changes from V1:
> - remove the buggy vq_error() in vhost_vq_more_avail().
> - leave vhost_enable_notify() untouched.
> 
> Changes from RFC V3:
> - small tweak on the code to avoid multiple duplicate conditions in
> critical path when busy loop is not enabled.
> - add the test result of multiple VMs
> 
> Changes from RFC 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 RFC 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_avail_empty()
>   vhost_net: basic polling support
> 
>  drivers/vhost/net.c        | 78 +++++++++++++++++++++++++++++++++++++++++++---
>  drivers/vhost/vhost.c      | 35 +++++++++++++++++++++
>  drivers/vhost/vhost.h      |  3 ++
>  include/uapi/linux/vhost.h |  6 ++++
>  4 files changed, 117 insertions(+), 5 deletions(-)
> 

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