Re: [Libevent-users] On which thread does bufferevent filter run, and when?

[Yucong Sun <sunyucong@xxxxxxxxx>]

On Wed, Jan 29, 2014 at 12:42 PM, Nick Mathewson <nickm@xxxxxxxxxxxxx> wrote:
> On Sun, Jan 26, 2014 at 2:58 AM, Yucong Sun <sunyucong@xxxxxxxxx> wrote:
>> Hi there,
>>
>> So I need to implement a zlib bufferevent filter to transparent
>> compress the data passing through a bufferevent. and but I want the
>> zlib work to be done in a separate thread to offload main loop.
>>
>> I currently call bufferevent_write() on main thread, would the filter
>> code executed immediately?  if I call  bufferevent_write() from
>> another thread, would the code be executed over that thread instead?
>> (locking is all properly setuped).
>
> The thread in which callbacks happen isn't currently specified by the
> documentation.  So it is allowed to change in future versions.
>
> In practice right now, I believe that filters are greedy: they consume
> all input as soon as you give it to them, and they process data from
> the underlying bufferevent as soon as it's read.  They do this in the
> same thread where the they receive the data.  (This is affected by
> watermarks on the underlying bufferevent.)

I don't quite understand this, what "callback" were you referring to?
When I call bufferevent_write() on a filtered socket based
bufferevent,   would the filters  A) get executed recurisvely on the
current thread/stack ? Or would B) it internally add some more events
to the loop and executes the filter code on next callback?

A) is better for me since I can arrange my call to bufferent_write()
onto another thread, thus offloading main loop. But for B) I can't
think of a way of doing that.

Thanks for the reply!

>
>> Also.. how does this play out with flush? is the filter code executed
>> only when calling flush?
>
> flushing on a filtered buffferevent makes the filter get called
> immediately with the provided mode.  Ordinarily, you use it to finish
> or clean-up a stream.  The filter code is also called with
> BEV_FINISHED or BEV_NORMAL as you add data to the buffer.
>
>
> hope this helps,
> --
> Nick
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