RFR: 8215159: Improve initial setup of system Properties

Peter Levart peter.levart at gmail.com
Tue Dec 11 09:02:24 UTC 2018

Hi Claes,

Haven't checked all changes yet, although it looks like a 
straightforward swap of Properties for HashMap for intermediary result, 
but I noticed the following in SystemProps:

  265             var cmdProps = new HashMap<String, 
String>((vmProps.length / 2) + Raw.FIXED_LENGTH);

The HashMap(int) constructor is different from Properties(int) in that 
for the former, the argument represents the lower bound on the initial 
size of the table (which is just a rounding of this parameter up to the 
nearest power of 2). The threshold where the table is resized is 
calculated as (initialCapacity rounded up to nearest power of 2) * 
loadFactor. The default load factor is 0.75 which means that the table 
will be resized in worst case after 3/4 * initialCapacity of elements 
are inserted into it. In order to guarantee that the table is not 
resized you have to pass (size * 4 + 2) / 3 to the HashMap constructor, 
where size is the number of elements added...

I hope I'm not misleading you, I just think this is how HashMap has been 
from the beginning. Peeking at HashMap code (line 693) it seems that it 
is doing that:

             float ft = (float)newCap * loadFactor;
             newThr = (newCap < MAXIMUM_CAPACITY && ft < 
                       (int)ft : Integer.MAX_VALUE);

newCap above is holding the initialCapacity constructor parameter 
rounded up to the nearest power of 2. newThr is the threshold at which 
the resize occurs.

The Properties(int) constuctor behaves differently as it passes the 
parameter directly to the underlying ConcurrentHashMap, which says:

      * @param initialCapacity The implementation performs internal
      * sizing to accommodate this many elements.

Regards, Peter

On 12/10/18 10:17 PM, Claes Redestad wrote:
> Hi,
> by inverting the order in which the internal property maps are created,
> we avoid some classloading and get a slightly more efficient code
> execution profile in System.initPhase1.
> Webrev: http://cr.openjdk.java.net/~redestad/8215159/jdk.00/
> Bug: https://bugs.openjdk.java.net/browse/JDK-8215159
> This reduces bytecode executed in initPhase1 from ~48k to ~36k. Not
> much by any measure, but minimizing System.initPhase1 is important since
> certain parts of the VM (JIT etc) are blocked from initializing until
> it's done.
> Thanks!
> /Claes

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