RFR 8005311: Add Scalable Updatable Variables, DoubleAccumulator, DoubleAdder, LongAccumulator, LongAdder

Remi Forax forax at univ-mlv.fr
Sun Jan 6 17:59:16 UTC 2013

On 01/06/2013 12:39 PM, Doug Lea wrote:
> On 01/05/13 18:42, Remi Forax wrote:
>> The code is not very java-ish,
> But is very consistent across j.u.c. As a convention,
> inline assignments to locals are used to hold field reads
> to visually ensure use of consistent snapshots. The C-like
> look-and-feel is a less important than is ability to simply
> check these cases by inspection.

I know that, I'm fine with inline assignment when needed.
But declaring variables on the first scope of a method or using a 
boolean variable to test if a try succeed is for me just a good way to 
make the code more obscure than it should.

>> Overall, I think there are too many lazy initializations.
>> Unlike HashMap, if a developer uses let say LongAccumulator it's because
>> AtomicLong doesn't work well,
>> so not having the array of cells initialized by default seems weird.
> You wouldn't say this if you were on a 256-way machine with
> millions of LongAdders, where only a thousand of them heavily
> contended :-)

I'm fine with uncontended/contended switch if the CAS on base fails but 
not with the fact that the array used in the contended case is not 
created when the thread local object is initialized.
BTW, why do you use an array of objects (Cell) that are padded enough to 
avoid false sharing instead of allocating an array big enough and to 
store the values at indexes that are far enough from each others, that 
should avoid one level of indirections.

Also, I've done a micro-test on my laptop (a modesr 4 cores machine), 
trying to calculate the maximum of one miliion values using 
LongAccumulator by 4 threads and the calls to Striped64::longAccumulate 
are never inlined,
longAccumulate is too big, a better segregation between the fast paths 
and the slow ones (that should be pushed in another method) should solve 
the issue.

> -Doug


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