Beta Branch add PHP_HUGEPAGES='n' variable in 123.09beta01

eva2000 · Dec 29, 2018

add PHP_HUGEPAGES='n' variable in 123.09beta01

- new PHP_HUGEPAGES='n' variable added which by default is disabled to control whether CentOS 7 + PHP 7+ builds enable Zend OpCache explicit Huge Pages supported setting opcache.huge_code_pages=1. If PHP_HUGEPAGES='y' set in persistent config file /etc/centminmod/custom_config.inc prior to centmin.sh menu option 5 recompile/update for PHP 7.x versions, then Zend OpCache explicit Huge Pages support setting opcache.huge_code_pages=1 is enabled + Kernel level settings are set appropriately.
- PHP Zend Opcache explicit Huge Pages support is not to be confused with Transparent Huge Pages
- Huge Page usage in PHP 7

Why use huge pages ?#
The concept is easy. If we make the OS Kernel use bigger page sizes, that means that more data can be accessed into one single page. That also means that we'll suffer from less TLB miss, once the page translation is stored into the TLB, because one translation will now be valid for more data. The Linux Kernel used to use default 4Kb page size on X86/64 architectures, but back in ~2.6.20, the Kernel evolved and has been added the huge page concept. A huge page is typically 512 times larger than a traditionnal page : it fits 2Mb. You can read many articles (1) about (2) huge pages (3) management in the linux Kernel, you'll then learn that usually the Kernel performs what's called transparent huge page mappings (Kernel 2.6.38) : it will map virtual memory onto traditionnal 4Kb pages, but will try sometimes (you can have hand on this) to merge contiguous pages into huge pages. This is especially done for the heap segment of processes, that can represent a very huge address space, but care should be taken as this memory may be given back to the OS (freed) by small chunks, hence wasting huge page space and probably forcing the Kernel to undo its job and shrink back a huge page into 512 smaller 4Kb pages.

However, the user process may itself ask for huge page based mappings specifically. If you are sure you're gonna fill-in the huge page space, then it is better to ask the Kernel to give you a huge page for such virtual memory page. Huge page means less pages to manage, that means smaller lists to browse by the Kernel in the process page table entry, and that means as well less entries in the crucial TLB cache. The system then becomes more efficient, more responsive and faster.

OPCache coming in help in PHP 7#
In PHP7, we worked very hard in improving memory efficiency. That means that we reworked crucial PHP internal structures, to make them fit into CPU caches more efficiently. We worked hard on data spacial locality, that means that more contiguous data fit into CPU caches, and then less memory accesses in main engine parts. This is the basic concept behind PHP 7 overall performances, and those are pretty easier to get under C than C++ (because C++ is a larger language, more complex and harder to master, together with its compiler).

In addition to that, we pushed the concepts of better memory efficiency even further by also experiencing Kernel huge pages. In PHP 7, the OPCache extension I already wrote about for PHP 5 has been added features to play with Kernel huge pages. Let's see that together.
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Branch: centminmod/centminmod

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Beta Branch add PHP_HUGEPAGES='n' variable in 123.09beta01

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Beta Branch add PHP_HUGEPAGES='n' variable in 123.09beta01

eva2000 Administrator Staff Member

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