ruby-core

Some comments:

Basically, great points.

(2014/08/28 18:10), Eric Wong wrote:

> The idea is to have a separate malloc
space for long-lived, WORM (write once, read many) data which may
increase memory sharing for forked processes.

Basically, "Write once" is important. Not for read many, isn't it?

Maybe there are several abilites for memory blocks:

- life time        short - long  - eternal
- write frequency  many  - a few - once
- read many        many  - a few - once - nothing
- size             variable size - fixed size

String buffer seems [lifetime: short, write: a few, read: many, size:
variable]. So String is out of scope.

Current rb_iseq_t is [lifetime: long, write: a few, read: a few (depends
on which method), size: fixed].

Bytecode refered from rb_iseq_t is [lifetime: long, write: once, read:
a few (depends...), size: variable].

fixed size block can be aggregate (with another space with mspace or
buffering technique) to avoid fragmentation.

So I assume the target of this API is such as bytecode, isn't it?


> The mspace API is similar to normal malloc:
> 
> 	malloc(size) => mspace_malloc(GET_VM()->cowspace, size)
> 	free(ptr) => mspace_free(GET_VM()->cowspace, ptr)
> 	...

Does mspace API supports to detect CoW breaking *writing*?
Or programmer should predicate that this area is WORM area?

Another consideration is that it seems depends on dlmalloc. How is
portability? (sorry I don't know about dlmalloc library more).

> There are rb_cow_(malloc/free/...) function wrappers as well
> as COW_(ALLOC/ZALLOC) macros to ease migrations.

I don't like _cow_ prefix because cow is purpose, does not represent the
ability of memory block. But it is not important before the merge.

> However, I haven't found any benefits, yet.  _Maintaining_
> CoW-friendliness is difficult in long-term so it might not be worth it
> (compared to overall memory reductions).

For ISeq, we need to change ISeq data structure to avoid write. I
believe this is my task.

(sorry for late)

-- 
// SASADA Koichi at atdot dot net