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Date: Tue, 14 Mar 2017 11:29:59 +0100
To: PHP internals list <internals@lists.php.net>
Subject: OPcache compilation performance regression in PHP 5.6/7 with huge
 classes
From: dz@heroku.com (David Zuelke)

Hi all,

There appears to be a performance regression in the CFG and DFA based =
optimization passes of OPcache in PHP 5.6+ when loading huge classes =
(such as those generated by Symfony's routing component) for the first =
time.

The issue does not occur on PHP 5.5. Tests below are with 5.6.30, 7.0.16 =
and 7.1.2 and default INI settings; I replicated the issue on both macOS =
and Linux.

Test file here (it's from an actual application, slightly anonymized, =
not a synthetic example): =
https://gist.github.com/dzuelke/fe867f55f09e0bf79ecefcc815b7fe92

Without OPcache, everything is fine in all versions:

$ time -p php -dopcache.enable_cli=3D0 hugeclass.php=20
real 0.10
user 0.09
sys 0.00

With OPcache on, things are suddenly much, much slower:

5.6:

$ time -p php -dopcache.enable_cli=3D1 hugeclass.php
real 3.23
user 3.21
sys 0.02

7.0:

$ time -p php -dopcache.enable_cli=3D1 hugeclass.php
real 1.76
user 1.73
sys 0.02

7.1:

$ time -p php -dopcache.enable_cli=3D1 hugeclass.php
real 4.01
user 3.98
sys 0.02

For comparison, 5.5 is as speedy as you'd expect it to be:

$ time -p php -dopcache.enable_cli=3D1 hugeclass.php=20
real 0.14
user 0.11
sys 0.02

If we switch off optimization passes 5 (CFG based) and 6 (DFA based, =
only in 7.1), everything is great again in all versions:

$ time -p php -dopcache.enable_cli=3D1 =
-dopcache.optimization_level=3D0x7FFFFFCF hugeclass.php
real 0.13
user 0.10
sys 0.02

For 5.6 and 7.0, pass 6 is not a thing, but in 7.1, we can inspect =
passes 5 and 6 separately.

Pass 5 (CFG based) already makes for a drastic performance hit in 7.1:

$ time -p php -dopcache.enable_cli=3D1 =
-dopcache.optimization_level=3D0x7FFFFFDF hugeclass.php
real 0.88
user 0.86
sys 0.01

But pass 6 (DFA based) is the one that causes the biggest slowdown in =
7.1:

$ time -p php -dopcache.enable_cli=3D1 =
-dopcache.optimization_level=3D0x7FFFFFEF hugeclass.php
real 3.29
user 3.24
sys 0.04

In all versions, subsequent loads from the cache (such as when running =
FPM or the built-in web server) are fast.

Is this slowness with a cold cache expected/accepted, or does that =
qualify as a bug?

David