OPcache compilation performance regression in PHP 5.6/7 with huge classes View as Markdown

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=0 hugeclass.php
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=1 hugeclass.php
real 3.23
user 3.21
sys 0.02

7.0:

$ time -p php -dopcache.enable_cli=1 hugeclass.php
real 1.76
user 1.73
sys 0.02

7.1:

$ time -p php -dopcache.enable_cli=1 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=1 hugeclass.php
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=1 -dopcache.optimization_level=0x7FFFFFCF 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=1 -dopcache.optimization_level=0x7FFFFFDF 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=1 -dopcache.optimization_level=0x7FFFFFEF 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

Just another poke to surface it, in case this should be merged into the 7.0 branch as well :)

https://bugs.php.net/bug.php?id=74250

Thanks for the fixes, Nikita!

Will they be merged to the PHP-7.0 branch as well?

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=0 hugeclass.php
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=1 hugeclass.php
real 3.23
user 3.21
sys 0.02

7.0:

$ time -p php -dopcache.enable_cli=1 hugeclass.php
real 1.76
user 1.73
sys 0.02

7.1:

$ time -p php -dopcache.enable_cli=1 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=1 hugeclass.php
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=1 -dopcache.optimization_level=0x7FFFFFCF 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=1 -dopcache.optimization_level=0x7FFFFFDF 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=1 -dopcache.optimization_level=0x7FFFFFEF 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?

Yes, this is a bug. Optimization should never be this slow.

From a quick perf run, the problem in the DFA pass is that TI is quadratic in the number of calls (as call lookup is implemented as a linear scan). This can be fixed with a more efficient map lookup.
The problem in the CFG pass is that a large buffer is zeroed repeatedly.

Nikita

Thanks; NEWS entry is missing though!

And it unfortunately just missed the 7.0.18 branch cutoff :(

Or can it be merged to PHP-7.0.18, given how problems would be caught in 7.1.4RC1 testing? ;)

Just another poke to surface it, in case this should be merged into the 7.0 branch as well :)

https://bugs.php.net/bug.php?id=74250

I've applied https://github.com/php/php-src/commit/d6f70f0ee3d5f595a9f6dadaabda834a1d47c662 to 7.0. The rest is not relevant.

Nikita