On Aug 23, 2008, at 00:59, Peter B. West wrote:
Jeremias Maerki wrote:
There's a lot of AND-ing of hash codes to determine the segments and
buckets and that's much easier to handle if you have a power of 2 and
can shift bits around.
On 22.08.2008 16:22:05 Peter B. West wrote:
Jeremias Maerki wrote:
Ok, here's the final patch I'd propose to fix the memory leak.
I've seen
no measurable performance degradation compared to the base
revision and
no multi-threading problems. The memory leak is fixed and the
number of
stale references remains low.
Feedback welcome.
What happens is SEGMENT_COUNT is not a power of 2? Or, what
obliges SEGMENT_COUNT to be a power of 2?
Peter
That was my assumption when I saw 'MASK', in which case is it not
safer to ensure that your value is in fact all ones?
MASK_BIT_COUNT = 5
SEGMENT_COUNT = 1<<MASK_BIT_COUNT
Hmm... safer? I do agree that this better reflects what is happening
in this case, but in general...
For a power as low as 5, there is only little difference, except that
bit-shifting may just be a tad faster; it used to be on older
hardware, and may still be so, due to the lack of bounds checking,
which brings us to the case of larger powers ( >= 32, or 64 ).
For larger exponents, using a mathematical rather than a bitwise
operator, if the result would be too large, one would at least get an
overflow error. With a bitwise operator, the leftmost bits will be
shifted into oblivion. No warning/error will be issued whatsoever.
Writing it in Hex, as 0x1F or (0x20 - 1) seems to be as common a
practice as any. Of course, if Java would allow one to express the
value in binary, that would make it even clearer...
But, now that you mention it, I think the relationship is turned
upside-down in the latest patch. Actually, the SEGMENT_COUNT should
depend on the SEGMENT_MASK (instead of the other way around).
Talking safety, maybe the following:
int SEGMENT_MASK = 0x1F; //binary ( 0000 0000 0000 0000 0000 0000
0001 1111 )
...
int SEGMENT_COUNT = SEGMENT_MASK + 1; //will cause an overflow error
if SEGMENT_MASK is set to Integer.MAX_VALUE or greater
Cheers
Andreas
