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: http://lwn.net/Articles/632590/ for previous test program and testing >>> resutls. >>> >>> Epoll manpage text: >>> >>> EPOLLEXCLUSIVE >>> Provides exclusive wakeups when attaching multiple epoll fds to a >>> shared wakeup source. Must be specified with an EPOLL_CTL_ADD operation. >>> >>> EPOLLROUNDROBIN >>> 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 > > Thanks, > > -Jason > -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
