make newFirstChunk more effective

[hicqu]

General Question

Here is a call graph about TiDB's CPU usage in attached file.

We can see newFirstChunk utilizes about 10% CPU of total.

I have tried to use a sync.Pool to cache *columns to improve it, but the result is not better. With the patch sync.Pool itself becomes the new bottle neck.

So please try to find a good wait to cache *columns to improve it.

[winoros]

What's your workload?
@zz-jason What about testing column pool in this environment?

[zz-jason]

There are many ways to implement a column pool. For example, sync.Pool, lock free list, channel based resource reuse.

• sync.Pool: the put and get operations is very fast, but the content in that pool is freed when golang GC is triggered.
• lock free list, channel: the put and get operations is slow compared with sync.Pool, but the content in the pool can be control by ourself.

There is still some problem to solve in the column pool implementation, I think we can leave this optimization in the future. For now, let's focused on chunk size control to improve the query latency.

[zz-jason]

There are some pool implementations and the benchmark results in this gist: https://gist.github.com/zz-jason/c98d942dddbb32c134c9cadffd983f61

This is a simple lock free list implementation: https://github.com/zz-jason/lockfree, the benchmark can be gained by running BenchmarkPushPop()

[hicqu]

I have tested sync.Pool but the result is bad. I prefer to use some lockfree collections or a channel, but I'm blocked on 1 thing: we must release *column manual and return it back to the pool. Could you help me? @zz-jason

[XuHuaiyu]

I'm blocked on 1 thing: we must release *column manual and return it back to the pool.

Can you describe more on this part?

[SunRunAway]

We can give sync.Pool a try again using go1.13beta. See golang/go#22950

[SunRunAway]

I used sysbench oltp_read_only to test on the master branch with go1.13beta1, it seems newFirstChunk is not a hot path. I'll close this issue and if someone finds another performance problem with newFirstChunk, feel free to reopen it.