Re: [Libevent-users] bufferevent_flush

[Nick Mathewson <nickm@xxxxxxxxxxxxx>]

On Mon, Jan 6, 2014 at 2:48 PM, Himanshu S <mail2himanshu@xxxxxxxxx> wrote:
> Happy new year folks,
>
> I am trying to chase down an issue which maybe be related to bufferevent. I
> am using bufferevent library to manage my TCP sessions. For TCP writes,
> instead bufferevent_write(), I use evbuffer_add_reference() to avoid copy of
> the data. TCP sessions exchange HELLO packets in addition to other relevant
> data. I am seeing a peculiar behavior.
>
>  A -------- B
>
> B is sending lots of data to A, while A is just sending Hello packets to B.
> When A is sending these hello packets, I see a huge latency (in order of
> seconds) on B. Through my internal debugs, I see that A is sending the hello
> packets when it is suppose to i.e. I see the debug on A that it called
> 'evbuffer_add_reference', but on B I don't see it. B periodically does check
> if there is any data from A as well.
>
> Q1) Does evbuffer_add_reference() call write() syscall synchronously or does
> it try to accumulate more data and then calls write()?

evbuffer_add_reference calls evbuffer_invoke_callbacks, which should
indeed trigger bufferevent_socket_outbuf_cb, which should (if
appropriate conditinos are met) enable the write event with
be_socket_add, which should (when the write event's callback there is
invoked) call bufferevent_writecb().  So if the socket really is
writeable, and you have a


This is no different than how evbuffer_add() to an outbuf treats
other, non-reference data.

To try to debug your latencies, you might want to instrument the
functions mentioned above to see when they're getting called, and if
they're behaving as expected.

best wishes,
-- 
Nick
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