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[PHP-DEV] OPcache compilation performance regression in PHP 5.6/7 with huge classes

Posted by 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=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


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On Tue, Mar 14, 2017 at 11:29 AM, David Zuelke <[email protected]> wrote:

> 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/dzuelk
> e/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
On 14 Mar 2017, at 16:39, Nikita Popov <[email protected]> wrote:
>
> On Tue, Mar 14, 2017 at 11:29 AM, David Zuelke <[email protected]> wrote:
>> 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.
>>
>> ....
>>
>> 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.

Alright, thanks! I opened a bug for it: https://bugs.php.net/bug.php?id=74250



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