JDK-6982173: Small problem causing thousands of wasted CPU hours

Jan Peter Stotz jpstotz at gmx.de
Wed Jan 23 20:05:11 UTC 2019


like many other I ran into bug JDK-698217 which is about 
AbstractSet.removeAll() and it's "aptitude" in wasting CPU time if you 
accidentally hit this bug. And there are hundred of developers hitting 
this bug every year.

I simply don't understand why there was no progress in 8 years, on a 
severe performance issue (a removeAll method on an efficient set that 
can require O(n^2)!) caused by code that was written to speed-up the 
removeAll implementation.

Which makes this bug worse is that it is triggered by the relative size 
of the current set compared to the collection passed as parameter.
Therefore for most developers this means not to use this buggy function 
at all (once they realized how worse it is).

IMHO there is a extreme simple workaround for avoiding this problem. The 
relevant part of the current implementation is as follows:

         if (size() > c.size()) {
             for (Object e : c)
                 modified |= remove(e);
         } else {
             for (Iterator<?> i = iterator(); i.hasNext(); ) {
                 if (c.contains(i.next())) {
                     modified = true;

The problem is that the code in the first if block only makes sense if 
the collections passed as parameter implements the remove(Object o) 
method in an efficient way - this means with a complexity of less than 
Java currently has no way to identify Collection implementations that 
satisfy this property we should at least implement a workaround that 
prevents the Java developers form unexpected "damage".
Which mean that the first if block should only be triggered if we are 
sure that it is faster than the second case (because removing an element 
from a Set is usually implemented efficiently).

Therefore I would propose a patch that no only compare the size of the 
current set and the given collection but also makes sure that the given 
collection is a Set or a Map:

     if (size() > c.size() && (c instanceof Set || c instanceof Map) {
          for (Object e : c)
              modified |= remove(e);
     } else {


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