[eleveldb][leo_backend_db] Impl iterater_move in batch

[mocchira]

As it turned out that interactions (to be precise, sending a message from an OS thread to an erlang scheduler thread) between erlang runtime and NIF are NOT negligible (hogged CPU rather than Disk I/O) especially if the number of call times is relatively large (For more details and evidences, see #974).
leo-project/eleveldb@61c1f5b has solved #974 by making the number of interactions from the number of records to 1 so let's take a step forward with this method, we can make this hack being applied to other iterator functions by adding a new I/F to eleveldb to iterate over items in a certain batch size, for example,

    %% This may return 30 items in BatchItems at most or less than 30 if it hits EOF.
    %% We can expect this function to decrease the CPU cost as it lowers the number of interactions between erlang runtime and NIF.
    {ok, BatchItems} = eleveldb:iterator_next_in_batch(IterRef, 30, []),

With this hack, we might be able to solve #710.
I'm going to try on the next working day.

[yosukehara]

@mocchira I've just confirmed your idea. I agree to it.

[mocchira]

@yosukehara Thanks for taking time. I will go ahead from now.

[mocchira]

After some experiments, I've decided to change the new I/F as below.

    %% This may return 30 items in BatchItems at most or less than 30 if it hits EOF.
    %% We can expect this function to decrease the CPU cost as it lowers the number of interactions between erlang runtime and NIF.
    {ok, BatchItems} = eleveldb:first_n(DBRef, 30),

Comparison to the previous I/F

• Pros
  • Much less code
  • Completely independent to the existing iterator related code base
• Cons
  • Use cases are narrowed down

However the iterating function we are using in leo_mq now is ONLY

• leo_backend_db_server:first_n

That said, just covering the first_n use case would solve most of our CPU hog problem for now.

[yosukehara]

@mocchira I've reviewed your idea, and I agree to deliver its function:

the first_n use case would solve most of our CPU hog problem

{ok, BatchItems} = eleveldb:first_n(DBRef, NumberOfItems)

[mocchira]

Confrimed this works fine. however unfortunately this improvement couldn't solve the #710 so I'll close this issue while leaving #710 open.