RFR 8017540: Improve multi-threaded contention behavior of BigInteger.toString() radix conversion cache

[Peter Levart]

Hi,

This version seems correct. Maybe just replace double volatile read at 
length re-check with a single one:


     private static BigInteger getRadixConversionCache(int radix, int 
exponent) {
         BigInteger[] cacheLine = powerCache[radix]; // volatile read
         if (exponent < cacheLine.length)
             return cacheLine[exponent];

         int oldLength = cacheLine.length;
         cacheLine = Arrays.copyOf(cacheLine, exponent + 1);
         for (int i = oldLength; i <= exponent; i++)
             cacheLine[i] = cacheLine[i - 1].pow(2);

         BigInteger[][] pc = powerCache; // volatile read again
         if (exponent >= pc[radix].length) {
             pc = pc.clone();
             pc[radix] = cacheLine;
             powerCache = pc; // volatile write, publish
         }
         return cacheLine[exponent];
     }


I like this, since it tries to avoid overwriting larger cacheLines with 
smaller ones when unexistent exponents are requested concurrently for 
same radix. This is good, since computation for larger exponents takes 
quadratically longer time (as Alan Eliasen points out) and possibility 
of concurrent threads stomping on each other increases. Re-reading and 
cloning powerCache reference at end of computation also takes care of 
cacheLines for other radixes that might have expanded while the 
computation was taking place. So the only overhead remaining is when 
concurrent threads are uselessly computing same results at same time. To 
avoid this, locking and waiting would have to be introduced which would 
complicate things.

Regards, Peter

On 06/26/2013 08:13 PM, Brian Burkhalter wrote:
> So do we have consensus on this version?
>
> Thanks for the lively "conversation."
>
> Brian
>
> On Jun 26, 2013, at 12:05 AM, Aleksey Shipilev wrote:
>
>> Yes, like that.
>>
>> -Aleksey
>>
>> On 26.06.2013, at 10:53, Dmitry Nadezhin <dmitry.nadezhin at gmail.com> wrote:
>>
>>>> We could check for the existing cacheLine.length right before installing
>>> the new one
>>>
>>> Do you mean something like this ?
>>>
>>> BigInteger getRadixConversionCache(int radix, int exponent) {
>>> BigInteger[] cacheLine = powerCache[radix]; // volatile read
>>> if (exponent < cacheLine.length)
>>>     return cacheLine[exponent];
>>>
>>> int oldLength = cacheLine.length;
>>> cacheLine = Arrays.copyOf(cacheLine, exponent + 1);
>>> for (int i = oldLength; i < exponent + 1; i++)
>>>     cacheLine[i] = cacheLine[i - 1].square();
>>>
>>> if (exponent >= powerCache[radix].length) { // volatile read again
>>>    BigInteger[][] pc = Arrays.copyOf(powerCache);
>>>    pc[radix] = cacheLine;
>>>    powerCache = pc; // volatile write, publish
>>> }
>>> return cacheLine[exponent];
>>> }