[tarantool-patches] Re: [PATCH 1/1] small: introduce static allocator

[Konstantin Osipov <kostja@tarantool.org>]

* Vladislav Shpilevoy <v.shpilevoy@tarantool.org> [19/05/04 09:38]:
> >>>
> >>> Shouldn't you align the address or provide an aligned alloc along
> >>> with the basic one?
> >>
> >> I thought about a function like static_aligned_alloc just like we
> >> have for region, but was not sure if we need it now. There are no
> >> code uses old tt_static_buf for data needed alignment. I hoped we
> >> could implement it on demand.
> >>
> >> But if you think we should implement it now, I can do that.
> >> Should I?
> > 
> > Previously tt_static_buf allocations were always aligned, since
> > the next alloc would always allocate at position 0 in the buffer.
> > Now you provide a general purpose alloc more or less, which can
> > start at any position in the buffer. I would simple ensure that
> > static_alloc() is intptr_t aligned. We can add an aligned alloc
> > later.
> If we align reserve() or alloc() by default, we would break the
> pattern, when we do big reserve of maximal size and then lots of
> small allocs of exact sizes. Works when we do not know how many
> bytes a big complex object will occupy.
> 
> We use that for mpstream with obuf, ibuf, region, and somewhere in
> SQL for pure region without mpstream. All our allocators guarantee
> now, that if you succeed at a big reserve(), you can consume this
> memory in multiple allocs, without gaps.
> 
> Additionally it contradicts with region API (the only allocator,
> providing aligned allocations) - it has separate reserve + alloc, and
> aligned_reserve + aligned_alloc.

On this argument... to use static alloc in mpstream, not aligning 
the return value is not enough.
You actually need something a batch alloc API, something 
like:
- static_alloc_begin_batch() - resets the buffer and sets the
position to 0,
- static_alloc_alloc_batch() - alloc memory, but return error if
  allocation would wrap around, since we need to make sure all
  allocation in a batch stay valid
- static_alloc_end_batch() - finish allocating memory within 
  a batch

Otherwise, how do you guarantee that a subsequent alloc used by
mpstream doesn't wrap the static buffer around and begins
overwriting it?


-- 
Konstantin Osipov, Moscow, Russia, +7 903 626 22 32