On Feb 27, 2014, at 2:50 PM, Peter Levart <peter.lev...@gmail.com> wrote:
> On 02/27/2014 09:22 AM, Paul Sandoz wrote: >> On Feb 27, 2014, at 12:51 AM, Brian Burkhalter <brian.burkhal...@oracle.com> >> wrote: >> >>> On Feb 26, 2014, at 1:53 PM, Paul Sandoz wrote: >>> >>>>> Thanks for the suggestion, Paul. Assuming it is correct, in what way >>>>> would this be a better approach? (I apologize for being obtuse.) >>>>> >>>> IMHO it is a simpler solution. It's a cache line that requires updating >>>> not the cache line*s*, as the current approach can potentially knock out >>>> cache lines for other radixes. > > Potentially yes, but not very likely, since the whole powerCache reference is > re-read again *after* the computation of cacheLine and then replaced with a > modified clone immediately after that. It should be noted what Alan Eliasen > already pointed-out in a discussion back in June 2013: > > http://mail.openjdk.java.net/pipermail/core-libs-dev/2013-June/018375.html > Yes, i was suggesting a CAS could be used to avoid replacing with a smaller cache line: while (!UnsafeHolder.casCacheLine(powerCache, radix, cacheLine, newCacheLine)) { cacheLine = powerCache[radix]; if (exponent < cacheLine.length) { break; } } but it can still result in two or more threads duplicating work as you say below, dunno how critical that is. I suppose if i was super paranoid about this i write some initialization code to warm the cache up :-) also being super paranoid perhaps there should be a limit of exponents that are cached? > ...so I think what could be improved in current implementation is the > potential and more likely useless computation of the same values for the same > radix by multiple concurrent threads. > Back then I presented an alternative solution which solves that: > > http://mail.openjdk.java.net/pipermail/core-libs-dev/2013-June/018457.html > > ...and as can be seen, does not use volatile read/write to publish > BigIntegers - that's why it is important that internal implementation of such > immutable classes like BigInteger is tolerant to unsafe publication... > Interesting, nice use of a linked list. Not sure that is safe publication of cacheLine into powerCache, the last element in a line could be observed as null, plus although it is unlikely, the compiler could shuffle up the array element store? Paul.
