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.

Any help would be very gratefully received!

(FYI - RHEL6 curl is 7.19, RHEL7 curl is 7.29)

Thanks, Alastair Scobie

School of Informatics
University of Edinburgh

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