reduce memory allocation when create ReadOptions object

[zhangjinpeng87]

No description provided.

[andelf]

better use Option<Box<Vec<T>>>

[zhangjinpeng87]

For what reason?

[andelf]

smaller size.

• Option<Vec> -> 4 usize (a pointer, a length, a capacity, a tag)
• Box<Vec<_>> -> 1 usize (a pointer)
• Option<Box<Vec>> -> 1 usize (a pointer, since Box impls NonZero optimization)

[andelf]

Option<Box<Vec<T>>> is used in rust implementation, called ThinVec<T>

[BusyJay]

Vec points a continuous memory on heap already, I think it's ridiculous to use Box again, which will allocate again.

[siddontang]

does it really cause the memory problem?

[andelf]

@siddontang no
@BusyJay this is the optimization for a normally-empty Vec. not ridiculous or something.

[BusyJay]

I don't think vec![] will allocate any memory on heap.

[andelf]

struct crocksdb_readoptions_t {
   ReadOptions rep;
   Slice upper_bound; // stack variable to set pointer to in ReadOptions
};

ReadOptions is a large struct.

better use singleton pattern?

[BusyJay]

A better way to do this is add a new C ABI to support snapshot read (event support upper bound). In that way, crocksdb_readoptions_t will be allocated on stack.

[andelf]

Wraps Options into Cow. export copy ctor.

[zhangjinpeng87]

In most cases the upper_bound is not set, this pr just use None instead of vec![].

[siddontang]

CI failed

[BusyJay]

So why replacing vec![] with None?

[zhangjinpeng87]

Close this pr since vec![] will not allocate any memory in heap.