• d-bugmail puremagic.com (29/47) Mar 14 2021 https://issues.dlang.org/show_bug.cgi?id=21716

d-bugmail puremagic.com writes:

https://issues.dlang.org/show_bug.cgi?id=21716

          Issue ID: 21716
           Summary: std.regex performance regression (additional GC
                    allocation)
           Product: D
           Version: D2
          Hardware: x86
                OS: Mac OS X
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: phobos
          Assignee: nobody puremagic.com
          Reporter: jrdemail2000-dlang yahoo.com

I have observed a regex related performance regression my tsv-utils package
when switching from LDC 1.24.0 to LDC 1.25.0. This corresponds to a performance
regression in DMD 2.095.


https://github.com/dlang/phobos/pull/7678.

A possible cause was identified by Petar Kirov in the issue comments
(https://github.com/dlang/phobos/pull/7678#issuecomment-787814712):

 At first glance, the suspect could be the two new delegates
 defaultFactoryImpl and matchOnceImpl. They may or may not
 cause GC allocations. If they are replaced with static nested
 functions, they would be guaranteed not to cause GC allocations.

Checking GC stats using "--DRT-gcopt=profile:1" indicates increased GC in the
program:

With DMD 2.094.2: 

 Number of collections:  0
 	Total GC prep time:  0 milliseconds
  	Total mark time:  0 milliseconds
 	Total sweep time:  0 milliseconds
 	Max Pause Time:  0 milliseconds
 	Grand total GC time:  0 milliseconds
 GC summary:    5 MB,    0 GC    0 ms, Pauses    0 ms <    0 ms

With DMD 2.095.1

 	Number of collections:  672
 	Total GC prep time:  7 milliseconds
 	Total mark time:  62 milliseconds
 	Total sweep time:  7 milliseconds
 	Max Pause Time:  1 milliseconds
 	Grand total GC time:  76 milliseconds
 GC summary:    5 MB,  672 GC   76 ms, Pauses   69 ms <    1 ms

The regular expression used in the tests is '[RD].*ION[0-2]', run against 14
million lines (one invocation per line). It's one of my standard benchmarks.

I am investigating further.

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