> On Apr 1, 2021, at 10:28 AM, Cong Wang <xiyou.wangc...@gmail.com> wrote:
>
> On Wed, Mar 31, 2021 at 11:38 PM Song Liu <songliubrav...@fb.com> wrote:
>>
>>
>>
>>> On Mar 31, 2021, at 9:26 PM, Cong Wang <xiyou.wangc...@gmail.com> wrote:
>>>
>>> From: Cong Wang <cong.w...@bytedance.com>
>>>
>>> (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.
How about we use a user space timer to trigger a BPF program (e.g. use
BPF_PROG_TEST_RUN on a raw_tp program); then, in the BPF program, we can
use bpf_for_each_map_elem and bpf_map_delete_elem to scan and update the
map? With this approach, we only need one syscall per period.
Thanks,
Song
