Description of problem:

Whilst porting an in-house application, which uses libcurl, from RHEL6 to RHEL7 we noticed that the time taken by libcurl to fetch a page had substantially increased. Even an attempt at fetching a non-existent page has increased from 10-20ms to around 150ms. This with just plain HTTP.

Our first thought was that the API has changed in some subtle way and the fault must be in our code, but then we thought to try measuring the performance of the curl tool itself. To our surprise, we found the same performance disparity.

Interestingly...

* Adding the DNS address of the HTTP server to /etc/hosts (and specifying files,dns in /etc/nsswitch.conf) does not improve things

* but, performance when using an IP address in the URL is fine (~ 10-20ms) for both RHEL6 and RHEL7

... which kind of suggests that the delay is in libcurl's DNS code rather than in the upstream resolver.

We did notice that when using DNS names in the URL, curl forks off a process to do the name lookup and then poll()s. There's no such fork when using an IP address in the URL. The poll() timesout after a period of time - we suspect this is the cause of the delay.



Version-Release number of selected component (if applicable):

RHEL 7.0

How reproducible:

Use the "curl" command to fetch a non-existent page off a working HTTP server.

Steps to Reproduce:
1. Setup an HTTP server, say myserver.mydomain.org
2. time curl http://myserver.mydomain.org/non-existent-page.html


Actual results:

Running curl on an RHEL 6.4 box, the time taken by curl is ~10-20ms. 
Running curl on an RHEL 7.0 box, the time taken by curl is ~150ms.

Expected results:

Time taken should be same under both OS releases.

Additional info:

This "bug" was reported to the Curl mailing list, and Daniel Stenberg suggested "this is simply because Redhat switched their build to the threaded resolver and in 7.29.0 we switched some libcurl internals which made us have that performance issue. I think this problem is solved in a more recent libcurl version."