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Postings from July 2010
Marvin Humphrey schrieb:
>> * Why the "+10"? Why have a extra minimum at all? Why "10" instead
>> of another number? Given your examples, for a large string the 10 is
>> a trivial fraction of the total and for a small string, the efficiency
>> either doesn't matter or the string quickly grows into a large string
>> anyway.
>
> That's similar to my analysis. I think it's more useful to round up to a
> multiple of the pointer size for small values.
I would rather align it to the pointer size, and keep an eye on the
pagesize.
I did similar tricks in Tie::CArray to get decent realloc
(array grow) performance.
http://cpansearch.perl.org/src/RURBAN/Tie-CArray-0.15/CArray.xs
#define PAGEBITS 11 /* we can also use 10 or 12, most system malloc to
12 ie 4096 */
#define MY_PAGESIZE (1 << PAGEBITS) /* 2048 byte is the size of a
fresh carray */
/* This is the to-tune part:
* The overall size should fit into a page or other malloc chunks.
* Leave room for "some" more items, but align it to the page size.
* Should small arrays (<100) be aligned at 2048 or smaller bounds?
* 10 => 2048-10, 2000 => 2048-2000, 200.000 => 2048
* len is the actual length of the array, size the itemsize in bytes.
*/
int freesize (int len, int size)
{
len *= size;
return max(MY_PAGESIZE-len, len - ((len >> PAGEBITS) << PAGEBITS))
/ size;
}
freesize is the slackspace to allocate additionally, and can be inlined.
grow by n:
ptr = saferealloc (ptr, len + freesize(len+n, itemsize));
--
Reini Urban
http://phpwiki.org/ http://murbreak.at/
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