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From Nick Wellnhofer <wellnho...@aevum.de>
Subject Re: [lucy-dev] Have Clownfish::Obj extend Perl SV
Date Thu, 11 Apr 2013 12:38:27 GMT
On 11/04/2013 02:48, Marvin Humphrey wrote:
> For the sake of argument, consider a brute-force alternative technique for
> rationalizing memory consumption: fill the SV object slot (self->ref.host_obj)
> during object allocation in `VTable_Make_Obj()`, rather than creating that SV
> object lazily when it's needed.  Unfortunately, that's all but certain to
> degrade both speed and memory consumption in the common case.  It sucks that
> we need two heap memory blocks for any object which is accessed from C-space
> and Perl-space.

We need even more allocations. An additional SV body for the inner 
object (since it's of type SvPVMG) and another SV head for the RV 
referencing the inner object. But instead of malloc, Perl uses a very 
efficient arena allocator for these structures. There's practically no 
memory overhead and an allocation usually just means popping a value off 
a linked list.

All these internals are illustrated here:

     http://cpansearch.perl.org/src/RURBAN/illguts-0.42/index.html

> But I guess you could say that the current system has some
> effective optimizations.

Yes, I just wrapped my head around all the details, and I think it's 
very nicely done. Here are some suggestions for further optimization:

Currently, we lazily allocate a Perl object when the refcount exceeds 4 
even if we don't really need it. There's a common technique to store 
flags in the least significant bits of an aligned pointer that could be 
employed here. Assuming that SVs are always allocated on even memory 
addresses, we could use the LSB of the lucy_ref_t union to signify 
whether the value is a refcount or a pointer to a host object. So we'd 
have all the other bits to store refcounts, and we'd only have to 
allocate a Perl object if we really need it.

If we allocate a Perl object then, I'd create the RV pointing to the 
inner object right away, store it in host_obj, and use it for 
refcounting. This saves a bit of memory and cycles if a Clownfish object 
is converted to a Perl object multiple times. Then we could also use 
sv_bless directly which is a bit slower than the code in 
S_lazy_init_host_obj but more forward-compatible (PL_sv_objcount will be 
deprecated in Perl 5.18, for example).

And, as Marvin suggested in his sample code, caching the stash pointer 
as a member of the VTable seems like a good idea.

Nick


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