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: 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 hmm....so 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. See: http://lkml.iu.edu/hypermail/linux/kernel/1201.1/02620.html http://marc.info/?l=linux-kernel&m=128638781921073&w=2 In the latter thread, Linus suggests adding it to the "requested events" field to poll: http://marc.info/?l=linux-kernel&m=128639416832335&w=2 So, I think that this series at least moves in that suggested direction. Thanks, -Jason -- 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
