Use select() syscall to avoid busy waiting

[diegode]

Currently it consumes a lot of CPU, it's a no-brainer i think.

[coveralls]

Coverage decreased (-0.09%) when pulling 8783764 on diegode:curl_multi_select into 81a917f on rmccue:master.

[rmccue]

Hmm, interesting. Can you explain exactly what this does please? :)

[diegode]

Well, busy-waiting is a technique in which a process repeatedly checks to see if a condition is true. In this case, the condition is "has any of the requests been answered?". This is in general a wasteful practice, so the OSes provide an array of system calls to avoid it.

For example, to wait 10 seconds you won't do:

$before = time();
while ($before+10 > time());

But instead:

sleep(10);

For this case, select() is a traditional solution, and cURL provides a wrapper through curl_multi_select(). It just blocks until one or more of the requests has I/O available, and you can set a timeout (1 second by default).

[rmccue]

For this case, select() is a traditional solution, and cURL provides a wrapper through curl_multi_select(). It just blocks until one or more of the requests has I/O available, and you can set a timeout (1 second by default).

Right, I understand how select works, but I thought curl_multi_exec automatically selected. Reading the documentation again, looks like I was wrong. :)

Thanks for this! I'll check it out and merge.

[staabm]

@rmccue any progress on this topic?

[rmccue]

Sorry, I've been pretty busy and didn't get around to trying this out yet. Will do.

[diegode]

ping @rmccue

[diegode]

Maybe we should add some checks as in http://stackoverflow.com/a/15173929

[TysonAndre]

I have a branch which independently has the exact same approach (to the exact same problem) in a slightly different location. See #284

https://secure.php.net/manual/en/function.curl-multi-select.php#refsect1-function.curl-multi-select-returnvalues

On failure, this function will return -1 on a select failure (from the underlying select system call).

We probably want to usleep() in the rare case (OS/PHP/curl setup?) that curl_multi_select returns -1, but this is something I'm in favor of.

[schlessera]

This PR was closed inadvertently when we changed the stable branch from master to stable. GitHub was supposed to smoothly adapt the PRs to point to the new branch, but this seems to have failed for the cases where the author's branch/fork of the PR is not available anymore.

[jrfnl]

PR recreated as #493