[DGD] COW arrays

[Raymond Jennings]

Just realized I started arguing instead of asking.  If I want to spend the
time to argue I'd be better off spending it implementing it myself.



On Sun, Jun 30, 2013 at 6:23 PM, Raymond Jennings <shentino at gmail.com>wrote:

>
>
>
> On Sun, Jun 30, 2013 at 2:33 PM, Felix A. Croes <felix at dworkin.nl> wrote:
>
>> Raymond Jennings <shentino at gmail.com> wrote:
>>
>> > Speaking strictly from a perspective of merit (I plan to work out
>> > implementation later if it's worth the trouble), would making duplicated
>> > arrays copy on write be worthwhile?
>> >
>> > Use case:
>> >
>> > For security reasons, it's often wise to pass arrays by value, ala
>> > array[..], to keep the callee from tampering with the contents of the
>> > master version.
>>
>> It would mean a minor slowdown and would probably not save memory,
>>
>
> Memory savings are only a fringe benefit here though.
>
> It's actually array copy overhead I'm targetting.
>
>
>> since arrays copied in the manner you describe are usually exported
>> to other objects, where they'll be copied anyway.
>
>
> Only if they are persistently saved.  I'm talking more about live
> tampering:
>
> arr = object->get_fragile_array();
> arr[someindex] = tampervalue
>
> where arr is a local variable (which means it doesn't get exported)
>
> Here, security demands that get_fragile_array() return a copy to avoid
> such tampering.
> I was wondering if having arrays COW'ed could optimize for the general
> case of well behaved callers that only read the array and don't write to it.
>
> Cases in point:  obj->query_inventory(), obj->query_archetypes()
>
> At least in my own codebase these are frequently called functions, and
> naturally I have to guard them with [..]'s to prevent malicious callers
> from tampering with them.  Though these arrays are rarely if ever modified
> in place by the caller, which means the array copy is wasted overhead.
>
> My hope was to use COW to elide array copies when not required.
>
> I think it's the same sort of logic dgd already uses with strings
>
> The minor slowdown
>> would come from the COW administration overhead, which of course would
>> also have an impact on memory usage.
>>
>
> Would this COW overhead be compensated for with a reduction in array
> copying overhead?
>
> A more interesting approach would be to make arrays copy-on-write in
>> all cases.  But that, of course, would be a change to LPC.
>>
>
> I think there's code out there that actually relies on array/mapping byref
> semantics anyway.
>
> Regards,
>> Felix Croes
>>
>
> I think the first think I'd do here is benchmark it as a configurable
> option.
>
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>