#1293: Array PMC freeze/thaw/visit broken
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Reporter: cotto | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: none | Version:
Severity: medium | Keywords:
Lang: | Patch:
Platform: |
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Comment(by whiteknight):
Sparse array algorithms represent a tradeoff: More efficient memory
storage for arrays that contain mostly default values in exchange for
more-expensive field access.
A sparse array is only a benefit in terms of storage space if the
following conditions are met:
- The array is very large
- The majority of items in the array are a default value
Even when it's a win in terms of storage efficiency, it's still going to
be more expensive _in all cases_ to access, modify, add, and remove items
from the array. If the storage is managed aggressively, it may be
performance neutral (or even a slight improvement in some cases) to do
copies and comparisons.
The Array PMC as it is implemented right now uses a linked list of
List_Chunk objects to implement it's storage. These List_Chunks are GCable
items, so the number of items in an array translates directly to increased
GC pressure. This is all not to mention that the Array PMC code is a large
example of how not to write a PMC following best practices.
If we are going to add sparse behavior to existing array types, I suggest
we:
- Not make sparse behavior the default.
- Add an "opt-in" ability to specify that an array should be treated as
sparse (buyer beware)
- Add an ability to check whether the array is currently in "sparse mode"
or "normal mode".
- Add a routine to check whether a given array would benefit from
sparseness
--
Ticket URL: <https://trac.parrot.org/parrot/ticket/1293#comment:3>
Parrot <https://trac.parrot.org/parrot/>
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