Memcached crashes when mem HWM hit for ephemeral buckets in 7.6.0

We are seeing issues in our KV "cache" perf test with 7.6.0. The test measures avg. ejections per second from a single ephemeral bucket, and with 7.6.0 we are seeing a reported 0 ejections per second:

Offending test runs:

1. http://perf.jenkins.couchbase.com/job/ares/36623/
2. http://perf.jenkins.couchbase.com/job/ares/36647/
3. http://perf.jenkins.couchbase.com/job/ares/36649/console (logs from this run: 172.23.133.13.zip - Update: there are no less than 5 crash dumps in here - presumably one for each time we lose all the items)

When looking at the cbmonitor comparison report between 7.5.0-4421 and 7.6.0-1295, we see that in 7.6 the curr_items is reduced to 0 several times during the test:

Examining the cluster during a test shows us that when we reach high mem watermark, in 7.6 we just seem to get rid of all active data and take no account of the low mem watermark:

The second issue (possibly a test issue, but not sure) is that the stats we are using to measure ejection rate are no longer working it seems. In our test we query the following stats directly from memcached (using a mcbp client):

• vb_active_auto_delete_count
• vb_pending_auto_delete_count
• vb_replica_auto_delete_count

It seems these are all just 0 the whole time.

Update: Couchbase Server crashes every time we reach high mem watermark it seems, so its a bit more of a serious issue...