Change default cache count from 65536 to 512

[lichunzhu]

What problem does this PR solve?

A son PR of #715.
Current default cache count is too big which can easily cause OOM of drainer.

What is changed and how it works?

The default size is changed from 65536 to 512.

Check List

Tests

• Unit test
• Integration test

Code changes

Side effects

Related changes

[lichunzhu]

/build

[suzaku]

In my opinion, worker-count * batch-size is not suitable as a default value, for the following reasons:

1. It may surprise the user. If a user finds that the downstream is quite far behind the upstream, he may try to make worker-count and batch-size larger and accidentally cause OOM.
2. There's a unit mismatch here. The unit for the buffer of drainer is the whole binlog, while the unit for batch-size is individual mutation in binlog. One binlog may contain many row mutations.

What we have learned so far is that 65536 may be too large for quite a few users, so maybe just explicitly use a smaller default value would be a safer way to solve the problem.

[lichunzhu]

@suzaku So what do you think of the right value? How about 512? I'm not so sure about this.
What's more, I think cache-binlog-count can avoid OOM in some degree, but a better way to avoid OOM is using cache-binlog-size of binlogItemCache since the count doesn't match the memory usage. Some binlog may very big which may cause OOM.

[suzaku]

@lichunzhu Note that cache-binlog-size can not avoid OOM in all cases, because the buffer is not the only data structure consuming memory. For example, if the downstream is TiDB/MySQL, we'll be exeucting SQLs in bulk and the binlogs not executed yet are stored in a slice which might take up a lot of memory if worker-count and batch-size is set to large numbers.

What I am suggesting is that, since we are facing a complicated problem with no perfect solution right now, we'd better just use a simple solution that can make OOM less likely.

[lichunzhu]

@suzaku I understand what you mean now. Then what do you think of the limit value? Is 512 OK?

[july2993]

@lichunzhu just change to 512 but don't add the config in SyncerConfig

[suzaku]

512 or anything less than or equal to 1024 should be a safe choice, because in loader we also have a buffered channel which is hard coded to be 1024 in length.

[lichunzhu]

@suzaku @july2993 I have modified defaultBinlogItemCount from 65536 to 512, PTAL.

[suzaku]

LGTM

[suzaku]

@lichunzhu Just writing 512 may be more readable than 1 >> 9?

[lichunzhu]

@lichunzhu Just writing 512 may be more readable than 1 >> 9?

Revised.

[july2993]

LGTM