[llvm] r193689 - Rehash but don't grow when full of tombstones.
Howard Hinnant
hhinnant at apple.com
Wed Oct 30 08:10:54 PDT 2013
Author: hhinnant
Date: Wed Oct 30 10:10:54 2013
New Revision: 193689
URL: http://llvm.org/viewvc/llvm-project?rev=193689&view=rev
Log:
Rehash but don't grow when full of tombstones.
This problem was found and fixed by José Fonseca in March 2011 for
SmallPtrSet, committed r128566. But as far as I can tell, all other
llvm hash tables retain the same problem: the bucket count can grow
without bound while size() remains near constant by repeated
insert/erase cycles that tend to fill the container with tombstones.
Here is a demo that has been reduced to a trivial case:
int
main()
{
llvm::DenseSet<unsigned> d;
for (unsigned i = 0; i < 0xFFFFFFF; ++i)
{
d.insert(i);
d.erase(i);
}
}
While the container size() never grows above 1, the bucket count grows
like this:
nb = 64
nb = 128
nb = 256
nb = 512
nb = 1024
nb = 2048
nb = 4096
nb = 8192
nb = 16384
nb = 32768
nb = 65536
nb = 131072
nb = 262144
nb = 524288
nb = 1048576
nb = 2097152
nb = 4194304
nb = 8388608
nb = 16777216
nb = 33554432
nb = 67108864
nb = 134217728
nb = 268435456
The above program currently consumes a few GB ram. This patch brings
the memory consumption down by several orders of magnitude, and keeps
the bucket count at 64 for the above test.
Modified:
llvm/trunk/include/llvm/ADT/DenseMap.h
Modified: llvm/trunk/include/llvm/ADT/DenseMap.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/DenseMap.h?rev=193689&r1=193688&r2=193689&view=diff
==============================================================================
--- llvm/trunk/include/llvm/ADT/DenseMap.h (original)
+++ llvm/trunk/include/llvm/ADT/DenseMap.h Wed Oct 30 10:10:54 2013
@@ -438,9 +438,8 @@ private:
this->grow(NumBuckets * 2);
LookupBucketFor(Key, TheBucket);
NumBuckets = getNumBuckets();
- }
- if (NumBuckets-(NewNumEntries+getNumTombstones()) <= NumBuckets/8) {
- this->grow(NumBuckets * 2);
+ } else if (NumBuckets-(NewNumEntries+getNumTombstones()) <= NumBuckets/8) {
+ this->grow(NumBuckets);
LookupBucketFor(Key, TheBucket);
}
assert(TheBucket);
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