On Wed, Mar 31, 2021 at 11:38 PM Song Liu <songliubraving@xxxxxx> wrote: > > > > > On Mar 31, 2021, at 9:26 PM, Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote: > > > > From: Cong Wang <cong.wang@xxxxxxxxxxxxx> > > > > (This patch is still in early stage and obviously incomplete. I am sending > > it out to get some high-level feedbacks. Please kindly ignore any coding > > details for now and focus on the design.) > > Could you please explain the use case of the timer? Is it the same as > earlier proposal of BPF_MAP_TYPE_TIMEOUT_HASH? > > Assuming that is the case, I guess the use case is to assign an expire > time for each element in a hash map; and periodically remove expired > element from the map. > > If this is still correct, my next question is: how does this compare > against a user space timer? Will the user space timer be too slow? Yes, as I explained in timeout hashmap patchset, doing it in user-space would require a lot of syscalls (without batching) or copying (with batching). I will add the explanation here, in case people miss why we need a timer. > > > > > This patch introduces a bpf timer map and a syscall to create bpf timer > > from user-space. > > > > The reason why we have to use a map is because the lifetime of a timer, > > without a map, we have to delete the timer before exiting the eBPF program, > > this would significately limit its use cases. With a map, the timer can > > stay as long as the map itself and can be actually updated via map update > > API's too, where the key is the timer ID and the value is the timer expire > > timer. > > > > Timer creation is not easy either. In order to prevent users creating a > > timer but not adding it to a map, we have to enforce this in the API which > > takes a map parameter and adds the new timer into the map in one shot. > > I think we don't have to address "creating a timer but not adding it to a map" > problem in the kernel. If the user forgot it, the user should debug it. Good point. Initially the timer is created in kernel-space, now it is in user space, so it is probably fine to create it without a map. But we would have to provide more syscalls for users to manage the timer, so using a map still has an advantage of not adding more syscalls. Thanks.
