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Re: blocking on dns resolving

From: Pedro Larroy <>
Date: Sun, 18 Dec 2011 23:41:10 +0100

On Sun, Dec 18, 2011 at 11:12 PM, Daniel Stenberg <> wrote:
> On Sun, 18 Dec 2011, Pedro Larroy wrote:
> I don't understand. That backtrace shows libcurl doing a poll() on a socket,
> yes and when the multi_socket API is used we should be able to do without
> it. We should fix that and we'll appreciate a patch that makes it work
> without the superfluous Curl_poll() call..
> You said it "just polls waiting for DNS queries to finish" and I don't see
> how your backtrace shows this. Can you perhaps explain with a few words what
> I'm not seeing?

My interpretation is that there's this poll call waiting for the dns
query to finish in the ares_channel associated with the current
curl_handle thus:

  * It will block the whole program until the current dns reply is
recieved, which goes against the paradigm of the multi interface usage
with asynchronous IO.
  * If that wasn't bad enough, the select has a timeout of 0.
  * I have already observed degradation and blocking which was very
detrimental and tracked down to this.

I think it's going to be difficult to fix since the multi api exposes
one socket perl curl handle, so it would have to expose all the
sockets of the dns resolution per handle to function properly, which
seems not trivial.


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Received on 2011-12-18