linux-kernel - Re: [PATCH V2 0/3] basic busy polling support for vhost

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Date:	Mon, 25 Jan 2016 11:00:05 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Mike Rapoport <rapoport@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 0/3] basic busy polling support for vhost_net



On 01/24/2016 05:00 PM, Mike Rapoport wrote:
> Hi Jason,
>
>> Jason Wang <jasowang <at> redhat.com> writes:
>>
>> 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.
> There were several conciens Michael raised on the Razya's attempt to add
> polling to vhost-net ([1], [2]). Some of them seem relevant for these
> patches as well:
>
> - What happens in overcommit scenarios?

We have an optimization here: busy polling will end if more than one
processes is runnable on local cpu. This was done by checking
single_task_running() in each iteration. So at the worst case, busy
polling should be as fast as or only a minor regression compared to
normal case. You can see this from the last test result.



> - Have you checked the effect of polling on some macro benchmarks?

I'm not sure I get the question. Cover letters shows some benchmark
result of netperf. What do you mean by "macro benchmarks"?

>
>> The maximum number of time (in us) could be spent on busy polling was
>> specified ioctl.
> Although ioctl is definitely more appropriate interface to allow user to
> tune polling, it's still not clear for me how *end user* will interact with
> it and how easy it would be for him/her.

There will be qemu part of the codes for end user. E.g. a vhost_poll_us
parameter for tap like:

-netdev tap,id=hn0,vhost=on,vhost_pull_us=20

Thanks

>
> [1] http://thread.gmane.org/gmane.linux.kernel/1765593
> [2] http://thread.gmane.org/gmane.comp.emulators.kvm.devel/131343
>
> --
> Sincerely yours,
> Mike.
>
>