dgraph-io / badger / 3011

61%

main: 61%

push

travis-ci

Fix deadlock in discard stats (#1070)

Fixes #1032

Currently discardStats flow is as follows:
* Discard Stats are generated during compaction. At the end, compaction routine updates these stats in vlog(vlog maintains all discard stats). If number of updates exceeds a threshold, a new request is generated and sent to write channel. Routine waits for request to complete(request.Wait()).
* Requests are consumed from write channel and written to vlog first and then to memtable.
* If memtable is full, it is flushed to flush channel.
*From flush channel, memtables are written to L0 only if there are less than or equal to NumLevelZeroTablesStall tables already.

Events which can lead to deadlock:
Compaction is running on L0 which has NumLevelZeroTablesStall tables currently and tries to flush discard stats to write channel. After pushing stats to write channel, it waits for write request to complete, which cannot be completed due to cyclic dependency.

Fix:
This PR introduces a flush channel(buffered) for discardStats. Compaction routine, will push generated discard stats to flush channel, if channel is full it just returns. This decouples compaction and writes. We have a separate routine for consuming stats from flush chan.

62 of 62 new or added lines in 2 files covered. (100.0%)

19324 of 24928 relevant lines covered (77.52%)

126583.63 hits per line