Long running php script with active file session blocks other requests from the same client. When the session is started php is trying to flock() the session file and if it is already locked by another process the script (and the whole apache process) hangs (or blocks). So all requests to php pages from the same client are effectivly serialized :( The lock/unlock should be probably placed only around read/write operations. The lock should not be held for the whole script lifetime...

BTW, the flock() is not released at all (!) when php script is called with ?=PHP_LOGO (where PHP_LOGO is a long string of hex digits, see sources)... check the main/main.c... with sessions.auto_start=1 the session IS started before you make PHPWRITE(image/gif) but it is NOT closed on return... That blocks other client's requests for a VERY long time... Just try to show php_info() with auto_start=1 (and expose_php=1 and enabled cookies in you browser)... see that browser is still active trying to download some image (Zend logo actually)... press STOP and shift-Reload... you are blocked... That problem is related to the first one in that while it can be solved different ways the clean solution is to unlock the file after read() and to try to lock the file again only before the write() (write() never happens with ?=PHP_LOGO/ZEND_LOGO/etc trick)...