Messages in this thread | | | Date | Mon, 20 Nov 2006 00:43:01 -0800 | From | Andrew Morton <> | Subject | Re: [take24 0/6] kevent: Generic event handling mechanism. |
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On Mon, 20 Nov 2006 11:25:01 +0300 Evgeniy Polyakov <johnpol@2ka.mipt.ru> wrote:
> On Sun, Nov 19, 2006 at 04:02:03PM -0800, Ulrich Drepper (drepper@redhat.com) wrote: > > Evgeniy Polyakov wrote: > > >>Possible solution: > > >> > > >>a) it would be possible to have a "used" flag in each ring buffer entry. > > >> That's too expensive, I guess. > > >> > > >>b) kevent_wait needs another parameter which specifies the which is the > > >> last (i.e., least recently added) entry in the ring buffer. > > >> Everything between this entry and the current head (in ->kidx) is > > >> occupied. If multiple threads arrive in kevent_wait the highest idx > > >> (with wrap around possibly lowest) is used. > > >> > > >> kevent_wait will not try to move more entries into the ring buffer > > >> if ->kidx and the higest index passed in to any kevent_wait call > > >> is equal (i.e., the ring buffer is full). > > >> > > >> There is one issue, though, and that is that a system call is needed > > >> to signal to the kernel that more entries in the ring buffer are > > >> processed and that they can be refilled. This goes against the > > >> kernel filling the ring buffer automatically (see below) > > > > > >If thread calls kevent_wait() it means it has processed previous entries, > > >one can call kevent_wait() with $num parameter as zero, which > > >means that thread does not want any new events, so nothing will be > > >copied. > > > > This doesn't solve the problem. You could only request new events when > > all previously reported events are processed. Plus: how do you report > > events if the you don't allow get_event pass them on? > > Userspace should itself maintain order and possibility to get event in > this implementation, kernel just returns events which were requested.
That would mean that in a multithreaded application (or multi-processes sharing the same MAP_SHARED ringbuffer), all threads/processes will be slowed down to wait for the slowest one.
> > >They all already imeplemented. Just all above, and it was done several > > >months ago already. No need to reinvent what is already there. > > >Even if we will decide to remove kevent_get_events() in favour of ring > > >buffer-only implementation, winting-for-event syscall will be > > >essentially kevent_get_events() without pointer to the place where to > > >put events. > > > > Right, but this limitation of the interface is important. It means the > > interface of the kernel is smaller: fewer possibilities for problems and > > fewer constraints if in future something should be changed (and smaller > > kernel). > > Ok, lets see for ring buffer implementation right now, and then we will > decide if we want to remove or to stay with kevent_get_events() syscall.
I agree that kevent_get_events() is duplicative and we shouldn't need it. Better to concentrate all our development effort on the single and most flexible means of delivery.
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