On 5/5/20 9:41 PM, Martin Buchholz wrote:
Hi Peter,
Are you saying guava has a tiny bug?
If it was just 1 too much when expected size is a multiple of 3 then
that would not be a bug, just sub-optimal calculation. And the same
calculation is performed also in JDK when a copy constructor is called
for example.
But I investigated further and what I found could be considered a bug.
Sometimes, the following expression:
(int) ((float) expectedSize / 0.75f + 1.0f)
...calculates a value that is not enough (due to floating point
arithmetic and conversion to int) to store the expectedSize elements
into the HashMap without re-hashing.
What HashMap does with initialCapacity parameter is to round it up to
nearest power of 2:
static int tableSizeFor(int cap) {
int n = -1 >>> Integer.numberOfLeadingZeros(cap - 1);
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY
: n + 1;
}
then it uses this as the initial backing table size. From that table
size it calculates the threshold value:
static int threshold(int cap) {
float ft = (float) cap * 0.75f;
return (cap < MAXIMUM_CAPACITY && ft < (float) MAXIMUM_CAPACITY ?
(int) ft : Integer.MAX_VALUE);
}
... and uses it as the max. number of elements that a HashMap can hold
before it is re-hashed. So I did the following test (comparing the
effectiveness of above formula with alternative (expectedSize*4+2)/3
formula):
public class HMTest {
static final int MAXIMUM_CAPACITY = 1 << 30;
static int tableSizeFor(int cap) {
int n = -1 >>> Integer.numberOfLeadingZeros(cap - 1);
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY
: n + 1;
}
static int threshold(int cap) {
float ft = (float) cap * 0.75f;
return (cap < MAXIMUM_CAPACITY && ft < (float) MAXIMUM_CAPACITY ?
(int) ft : Integer.MAX_VALUE);
}
public static void main(String[] args) {
for (int expectedSize = 0; expectedSize < (Integer.MAX_VALUE -
2) / 4; expectedSize++) {
int cap1 = (int) ((float) expectedSize / 0.75f + 1.0f);
int cap2 = (expectedSize * 4 + 2) / 3;
int ts1 = tableSizeFor(cap1);
int ts2 = tableSizeFor(cap2);
int th1 = threshold(ts1);
int th2 = threshold(ts2);
if (th1 < expectedSize || th2 < expectedSize) {
System.out.printf("%d: (%d, %d, %d)%s (%d, %d, %d)%s\n",
expectedSize,
cap1, ts1, th1, (th1 < expectedSize) ? "!" : " ",
cap2, ts2, th2, (th2 < expectedSize) ? "!" : " "
);
}
}
}
}
And what this prints is the following:
25165825: (33554432, 33554432, 25165824)! (33554434, 67108864, 50331648)
50331649: (67108864, 67108864, 50331648)! (67108866, 134217728, 100663296)
50331650: (67108864, 67108864, 50331648)! (67108867, 134217728, 100663296)
100663297: (134217728, 134217728, 100663296)! (134217730, 268435456,
201326592)
100663298: (134217728, 134217728, 100663296)! (134217731, 268435456,
201326592)
100663299: (134217728, 134217728, 100663296)! (134217732, 268435456,
201326592)
100663300: (134217728, 134217728, 100663296)! (134217734, 268435456,
201326592)
201326593: (268435456, 268435456, 201326592)! (268435458, 536870912,
402653184)
201326594: (268435456, 268435456, 201326592)! (268435459, 536870912,
402653184)
201326595: (268435456, 268435456, 201326592)! (268435460, 536870912,
402653184)
201326596: (268435456, 268435456, 201326592)! (268435462, 536870912,
402653184)
201326597: (268435456, 268435456, 201326592)! (268435463, 536870912,
402653184)
201326598: (268435456, 268435456, 201326592)! (268435464, 536870912,
402653184)
201326599: (268435456, 268435456, 201326592)! (268435466, 536870912,
402653184)
201326600: (268435456, 268435456, 201326592)! (268435467, 536870912,
402653184)
402653185: (536870912, 536870912, 402653184)! (536870914, 1073741824,
2147483647)
402653186: (536870912, 536870912, 402653184)! (536870915, 1073741824,
2147483647)
402653187: (536870912, 536870912, 402653184)! (536870916, 1073741824,
2147483647)
402653188: (536870912, 536870912, 402653184)! (536870918, 1073741824,
2147483647)
402653189: (536870912, 536870912, 402653184)! (536870919, 1073741824,
2147483647)
402653190: (536870912, 536870912, 402653184)! (536870920, 1073741824,
2147483647)
402653191: (536870912, 536870912, 402653184)! (536870922, 1073741824,
2147483647)
402653192: (536870912, 536870912, 402653184)! (536870923, 1073741824,
2147483647)
402653193: (536870912, 536870912, 402653184)! (536870924, 1073741824,
2147483647)
402653194: (536870912, 536870912, 402653184)! (536870926, 1073741824,
2147483647)
402653195: (536870912, 536870912, 402653184)! (536870927, 1073741824,
2147483647)
402653196: (536870912, 536870912, 402653184)! (536870928, 1073741824,
2147483647)
402653197: (536870912, 536870912, 402653184)! (536870930, 1073741824,
2147483647)
402653198: (536870912, 536870912, 402653184)! (536870931, 1073741824,
2147483647)
402653199: (536870912, 536870912, 402653184)! (536870932, 1073741824,
2147483647)
402653200: (536870912, 536870912, 402653184)! (536870934, 1073741824,
2147483647)
So as you see, for expectedSize < (Integer.MAX_VALUE - 2) / 4 (where the
alternative formula does not experience overflow and is enough for
Naoto's case) all miscalculations are due to the JDK/Guava formula which
in those cases calculates a value that is less than alternative
formula's value and too small to adequately pre-size the HashMap table.
Voila, we have some bugs to fix or I am doing something wrong here.
Regards, Peter
On Tue, May 5, 2020 at 12:12 PM Peter Levart <peter.lev...@gmail.com
<mailto:peter.lev...@gmail.com>> wrote:
Hi Martin,
On 5/5/20 8:26 PM, Martin Buchholz wrote:
See related:
https://guava.dev/releases/23.0/api/docs/com/google/common/collect/Maps.html#newHashMapWithExpectedSize-int-
This is basically the same calculation (or at least gives same
result) as Naoto did (without the max part):
Naoto: (int)(expectedSize / 0.75f) + 1
Guava: (int) ((float) expectedSize / 0.75F + 1.0F)
but in case expectedSize is a multiple of 3, it gives the result
which is 1 more than needed. If what is needed is also a power of
2, then twice the needed space is allocated in the HashMap backing
table.
Regards, Peter
On Tue, May 5, 2020 at 11:03 AM <naoto.s...@oracle.com
<mailto:naoto.s...@oracle.com>> wrote:
And here is the fix. Please review.
http://cr.openjdk.java.net/~naoto/8244459/webrev.00/
Naoto
On 5/5/20 10:25 AM, naoto.s...@oracle.com
<mailto:naoto.s...@oracle.com> wrote:
> Hi Peter,
>
> You are correct. Thanks. I'll remove that initial value of 16.
>
> Naoto
>
> On 5/5/20 9:37 AM, Peter Levart wrote:
>> Hi Naoto,
>>
>> On 4/30/20 12:18 AM, naoto.s...@oracle.com
<mailto:naoto.s...@oracle.com> wrote:
>>> Hello,
>>>
>>> Please review this small fix to the following issue:
>>>
>>> https://bugs.openjdk.java.net/browse/JDK-8244152
>>>
>>> The proposed changeset is located at:
>>>
>>> https://cr.openjdk.java.net/~naoto/8244152/webrev.00/
>>>
>>> The hash map used there didn't have initial capacity,
even though the
>>> exact numbers are known.
>>
>>
>> Well, it has to be calculated 1st (countTokens), but I
guess this pays
>> off when HashSet (the backing HashMap) does not have to be
rehashed then.
>>
>> The expression you use:
>>
>> Math.max((int)(tokens.countTokens() / 0.75f) + 1, 16)
>>
>> ...has a minimum value of 16. Why is that? 16 is just
HashMap's
>> default initialCapacity if not specified explicitly. But
if you only
>> want to store say 1 entry in the map, you can specify 2 as
>> initialCapacity and HashMap will happily work for such
case without
>> resizing.
>>
>>
>> So you could just use:
>>
>> (int)(tokens.countTokens() / 0.75f) + 1
>>
>> And even this expression is sometimes overshooting the
minimal
>> required value by 1 (when # of tokens is "exact" multiple
of 0.75f,
>> say 6). I think the following could be used to optimally
pre-size the
>> HashMap with default load factor 0.75:
>>
>> (tokens.countTokens() * 4 + 2) / 3
>>
>>
>> Regards, Peter
>>
>>>
>>> Naoto
