Hi Rasmus, > -----Original Message----- > From: Rasmus [mailto:ras...@lerdorf.com] > Sent: Sunday, March 20, 2016 5:29 AM > To: php-internals <internals@lists.php.net> > Subject: [PHP-DEV] Huge Pages > > We have a bit of a problem with how Linux handles huge pages when you run > out of them. > > I've been discussing it with Dmitry and he recently committed a way to disable > Huge Pages in the main allocator to master: > > > https://github.com/php/php- > src/commit/945a661912612cdecd221cd126feda7d4335c33c > > Unfortunately it looks like Linux is very unhappy when a CoW happens to a huge > page allocated in its parent process and it tries to re-map this page. If > there are > no available huge pages you get random seg faults/bus errors and everything > blows up. I don't understand why Linux can't fall back to a regular page here > in > this case, but it doesn't. It seems like an OS bug to me, but nonetheless this > affects mod_php/php-fpm since we have a huge page allocated before the fork, > but it also affects any cli script that does a pcntl_fork(). As far as I can > tell there > is no way to fix it. We could try to make sure we never do a MAP_HUGETLB prior > to the fork in mod_php/php-fpm but that still doesn't solve the pcntl_fork() > case. > > Anatol, we should merge Dmitry's patch into PHP-7.0 and further, I think we > should flip the default to zend_mm_use_huge_pages = 0 in it. > > If you have well-behaved code that never tries to allocate a lot of memory you > can turn on huge pages, but I think we should default to safety here. Those > SIGBUS storms are quite alarming and hard to figure out. > > There have also been some other issues with huge pages as per these > bugs: > > https://bugs.php.net/70984 > https://bugs.php.net/71272 > https://bugs.php.net/71355 > > This is very much an expert feature that we need to document better and people > should know what they are stepping in when they turn this feature on. > Dmitry has already merged this patch into the 7.0 branch. I also know from Remi that Fedora doesn't play well with the huge pages. Huge pages require special system settings which are usually not enabled in OSes by default. Having it off by default in PHP would therefore not cause any functional disadvantage while addressing the stability concerns. Thus reversing the default setting sounds feasible, it could be already done for the upcoming 7.0.5 if there are no objections.
Regards Anatol -- PHP Internals - PHP Runtime Development Mailing List To unsubscribe, visit: http://www.php.net/unsub.php
