Change q050's nextList from q075 to q100, to reduce the call stack depth of method PoolChunkList.add(chunk)

[laosijikaichele]

Motivation:

In PoolArena, the q050's nextList should be q100.

Because when we call method PoolChunkList.add(chunk) to move a chunk, if a chunk moved from q050 to q075, it will eventually move to q100 after the call finished.

So we can make q050.nextList = q100 in PoolArena's constructor, to reduce the call stack depth of method PoolChunkList.add(chunk).

Modification:

Change q050's nextList from q075 to q100 in PoolArena's constructor.

Result:

Reduced the call stack depth of method PoolChunkList.add(chunk).

[normanmaurer]

@chrisvest @franz1981 @trustin can you please have a look

[chrisvest]

when we call method PoolChunkList.add(chunk) to move a chunk, if a chunk moved from q050 to q075, it will eventually move to q100 after the call finished.

I don't understand why this is a given.

If the call stack depth is a problem, we can make the add method iterative instead of recursive.
If the 75-percentile list serves little purpose (it's last resort in allocation order), we could perhaps remove it entirely?

[normanmaurer]

when we call method PoolChunkList.add(chunk) to move a chunk, if a chunk moved from q050 to q075, it will eventually move to q100 after the call finished.

I don't understand why this is a given.

If the call stack depth is a problem, we can make the add method iterative instead of recursive. If the 75-percentile list serves little purpose (it's last resort in allocation order), we could perhaps remove it entirely?

I agree... IMHO we can just close this

[laosijikaichele]

@chrisvest

I don't understand why this is a given.

Because under current(4.1 branch) implementation, for the allocation order, when the chunk moving from q050 -> q075->q100, it will NEVER stop moving on q075, which means the chunk will move to q100 once it leaves q050, so the q075 is just a temporary transit and thus is not needed in allocation order.

If the call stack depth is a problem, we can make the add method iterative instead of recursive.

The gain of reducing only one stack depth is trivial, but it is still a positive trivial change, IMHO.
We can skip q075 in allocation order by simply not link it in PoolArena's constructor, without need to modify the add method, I think this is the most simple way to approach.

If the 75-percentile list serves little purpose (it's last resort in allocation order), we could perhaps remove it entirely?

We can't remove the q075 entirely, because although the q075 is not needed in allocation order, but it is still needed in de-allocation order, as the following code shows in PoolArena's constructor:

q100.prevList(q075);
q075.prevList(q050);

The following diagrams show the chunkList nodes links.

Before this PR:

After this PR: