Typical case I've used it was to slice things for columnar display. I end up
using the entire array, but just in N chunks of M items at a time.
I don't recall ever taking a slice and discarding the rest.
fwiw.
-Bob
On Oct 26, 2011, at 9:28 AM, Thomas E Enebo wrote:
> Does anyone do a lot of sliciing (e.g. things like [1...100]) in their
> apps. Are they big arrays? Does anyone know a library which does?
>
> -Tom
>
> On Tue, Oct 25, 2011 at 6:46 PM, Charles Oliver Nutter
> <[email protected]> wrote:
>> Long ago when the world was new, Marcin Mielzynski modified our Array
>> implementation to be copy-on-write like MRI. And there was much
>> rejoicing.
>>
>> COW helps methods like slice and dup that would normally need to make
>> a copy of all or part of the source array by wrapping the original
>> array in "shared" mode. Subsequent modifications force the original
>> contents to be copied into a new array, but for read-only subsets
>> (especially if they're big) it's a win.
>>
>> However, it suffers from a big down side: if you have a really large
>> array holding onto many large objects, and you slice out a small
>> subset of that array, the original contents stick around. This is like
>> a pseudo-leak, since the memory the rest of the array is rooting never
>> gets collected. We've patched around this in a few key methods.
>>
>> So today I thought I'd try removing COW from Array completely and
>> comparing numbers. Here's the result and the patch:
>>
>> https://gist.github.com/beab95be678c519e8a24
>>
>> The bottom line is that it definitely has an impact. Any operation
>> that would create a new view of N elements of the original array goes
>> from being O(1) (the time needed to wrap the old array contents in a
>> new object) to O(N) (the time needed to copy all those elements into a
>> new array.
>>
>> I post this here so anyone interested in testing it against real code
>> or real applications can do so. I just used some simple benchmarks
>> from Rubinius, which certainly aren't going to behave like a typical
>> app. Maybe the degradation is small enough for real apps that the
>> GC/memory improvements would be worth it?
>>
>> - Charlie
>>
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>
>
>
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