[erlang-questions] gen_tcp nonblocking send

Igor Ribeiro Sucupira <>
Thu May 1 21:07:39 CEST 2008

On Thu, May 1, 2008 at 3:16 PM, Valentin Micic <> wrote:
> As you've said -- it is a workaround. Consider that under some circumstances
>  such workaround may cause problems -- think of it this way: You're not
>  really solving the problem, just shifting it elsewhere.
>  Under normal circumstances (as you've outlined), if sender and receiver are
>  not in some kind of "synchronous" engagement, in other words, if your next
>  send does not depend on receiving something from a remote peer; and you have
>  a "sluggish" remote receiver, your process would block once local buffer is
>  full. If you change this by introducing intermediate server, as suggested --
>  call it a REALY; you are not changing the fact that this RELAY will block
>  when local sending buffer gets full. However, as the sending process is now
>  sending "normal" Erlang messages, it will have no way of figuring out that
>  RELAY's buffer is full, thus, it will keep sending, eventually clogging
>  RELAY's process message queue.

This is not the kind of solution we were suggesting to Daniel.
He's got a continuous flux of update messages, of which he wants to
send only the most recent to the server. We don't want him to
introduce a relay that will just forward all messages it receives. The
idea is to have a relay that will take care of sending only the most
recent message once the last send returns.
(In fact, I implemented that yesterday, to make sure it was easy  :p)

An example:
- Update {k1, 17} sent to relay process.
- Relay process starts sending (using another process) {k1, 17}.
- Update {k1, 18} sent to relay process.
- Update {k1, 19} sent to relay process ({k1, 18} is discarded).
- Update {k1, 20} sent to relay process ({k1, 19} is discarded).
- Sending of {k1, 17} finishes.
- Sending of {k1, 20} starts.

That won't cause any problems to any process message queue.


> In my experience, once process has a few
>  hundred thousands of such messages in its queue, Erlang scheduling starts
>  having problems (actually, this is largely due to the CPU utilization, but
>  there is a relationship), hence you are degrading performance of other
>  processes as well.
>  OTOH, if one assumes that O_NDELAY would return immediately once the local
>  buffer is full, this would help solving the problem you're referring to, by
>  allowing a programmer to find the most suitable solution to a given
>  problem - anything from immediate disconnection/reconnection to some form of
>  internal buffering (which, mind you, cost only in terms of memory, and not
>  so much in CPU cycles).
>  I could understand the problems around introduction of O_NDELAY, e.g. how to
>  communicate to client that only N octets were sent, and that rest should be
>  resent. Well, maybe introducing a separate function, say
>  gen_tcp:non_blocking_send,  that would return a binary consisting of unsent
>  octets (or empty binary if all has been sent), might not be so far fetched
>  (hint, hint).
>  V.
>  ----- Original Message -----
>  From: "Daniel Ginsburg" <>
> To: "Valentin Micic" <>
>  Cc: "Per Hedeland" <>; <>
>  Sent: Thursday, May 01, 2008 4:03 PM
>  Subject: Re: [erlang-questions] gen_tcp nonblocking send
> > On 01.05.2008 17:50 Valentin Micic wrote:
>  >
>  >> Does this mean that O_NDELAY is not supported in Erlang? Not even as an
>  >> undocumented feature? ;-)
>  >>
>  >
>  > Internally, sockets are non-blocking (see
>  > erts/emulator/drivers/common/inet_drv.c), but as far as I understand,
>  > gen_tcp and all the underlying libraries like prim_inet expose only
>  > blocking send API to erlang processes.
>  >
>  > There's nice workaround, as Per suggested. I tried it, and it works like
>  > charm (thanks again, Per!).
>  >
>  > There's also unresolved problem with hot code upgrade for blocked process,
>  > but for me this is a minor issue.
>  >
>  > --
>  > dg
>  >
>  _______________________________________________
>  erlang-questions mailing list
>  http://www.erlang.org/mailman/listinfo/erlang-questions

More information about the erlang-questions mailing list