Re: [PATCH v2 0/2] Add epoll round robin wakeup mode

From: Jason Baron
Date: Tue Feb 17 2015 - 22:15:58 EST

On 02/17/2015 04:09 PM, Andy Lutomirski wrote:
> On Tue, Feb 17, 2015 at 12:33 PM, Jason Baron <jbaron@xxxxxxxxxx> wrote:
>> On 02/17/2015 02:46 PM, Andy Lutomirski wrote:
>>> On Tue, Feb 17, 2015 at 11:33 AM, Jason Baron <jbaron@xxxxxxxxxx> wrote:
>>>> When we are sharing a wakeup source among multiple epoll fds, we end up with
>>>> thundering herd wakeups, since there is currently no way to add to the
>>>> wakeup source exclusively. This series introduces 2 new epoll flags,
>>>> EPOLLEXCLUSIVE for adding to a wakeup source exclusively. And EPOLLROUNDROBIN
>>>> which is to be used in conjunction to EPOLLEXCLUSIVE to evenly
>>>> distribute the wakeups. This patch was originally motivated by a desire to
>>>> improve wakeup balance and cpu usage for a listen socket() shared amongst
>>>> multiple epoll fd sets.
>>>> See: for previous test program and testing
>>>> resutls.
>>>> Epoll manpage text:
>>>> Provides exclusive wakeups when attaching multiple epoll fds to a
>>>> shared wakeup source. Must be specified with an EPOLL_CTL_ADD operation.
>>>> Provides balancing for exclusive wakeups when attaching multiple epoll
>>>> fds to a shared wakeup soruce. Depends on EPOLLEXCLUSIVE being set and
>>>> must be specified with an EPOLL_CTL_ADD operation.
>>>> Thanks,
>>> What permissions do you need on the file descriptor to do this? This
>>> will be the first case where a poll-like operation has side effects,
>>> and that's rather weird IMO.
>> So in the case where you have both non-exclusive and exclusive
>> waiters, all of the non-exclusive waiters will continue to get woken
>> up. However, I think you're getting at having multiple exclusive
>> waiters and potentially 'starving' out other exclusive waiters.
>> In general, I think wait queues are associated with a 'struct file',
>> so I think unless you are sharing your fd table, this isn't an issue.
>> However, there may be cases where this is not true? In which
>> case, perhaps, we could limit this to CAP_SYS_ADMIN...
> There's also SCM_RIGHTS, which can be used in conjunction with file
> sealing and such.
> In general, I feel like this patch series solves a problem that isn't
> well understood and does it by adding a rather strange new mechanism.
> Is there really a problem that can't be addressed by more normal epoll
> features?
> --Andy I dug through some of the Linux archives a bit and this
problem seems to crop up every so often without resolution.
So I do believe that its an issue that ppl are more generally
interested in.


In the latter thread, Linus suggests adding it to the "requested events"
field to poll:

So, I think that this series at least moves in that suggested direction.


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