What could be causing high cpu in google spanner databases? (unresponsive table)

Question

I'm facing an issue where 2 out of 10 spanner databases are showing a high CPU usage (above 40%) whereas the others are around %1 each, with almost identical or more data.

I notice one of our tables has become "unresponsive" no queries work against it. We shutdown all apps that connect to those dbs, and we also deleted all current sessions using gcloud sessions list and then gcloud session delete.

However the table is still unresponsive. A simple select like select id from mytable where name = 'test' is not responding (when tested from an app, and also from gcloud web interface), it only happens with that table, which has only a few columns with normal data and no more than 2000 records. We identified the query that could have been the source of the problem, however the table seems to be locked (only count(*) without any where clause works).

I was wondering if there is any way to "unlock" the table, kill those "transactions" that might be causing the issue, or restart those specific spanner databases, or in the worst case scenario restarting the spanner instance.

I have seen the monitoring high cpu documentation, but even if we can identify the cpu is high, we don't really know how to restart or make it back to normal before reviewing the query/ies that could have caused the issue (if that was the case).

Answer 1

High CPU can be caused by user initiated queries, from different types of operations. It is important to notice that your instance is where you allocate resources to be used by the underlying Cloud Spanner databases. This means, that if all of your databases are in the same instance and if some of your databases are hogging the CPU, all your other databases will struggle.

In terms of a locked table, I would be very surprised if a deadlock is the problem here, since Spanner mitigates those issues using "wound-wait" algorithm. What I suspect might be happening is that a long time is necessary to perform the query in that table and it times out. It would be nice to investigate a bit more on your problem:

• How long did you wait for your query on the problematic table (before you deemed to be stuck)? It might be a problem where your query just takes long and you are timing out before getting the results. It might be a problem where there are hotspots in your table and transactions are getting aborted often, preventing you from getting results.

• What error did you get when performing the query on the table? The error code can tell you more about what might be happening.

• Have you tried doing a stale read on the table to see if any data is returned? If lock contention is the problem, this should succeed and return results faster for you. Thus, you can further investigate the lock usage with the statistics table (as shown below).

• Inspect query statistics: you can list the queries with the highest CPU usage, find the average latency for a query and find out the queries that timeout the most. There is much more you can do, as seen here. You can also view the query statistics in cloud console. What I would verify is, by reducing the overall CPU, does your query come back without any issues? You might need more resources if so. Or you might need to reduce hotspots in your database.

• Inspect Lock statistics: you can investigate lock conflicts in your rows and tables. I think that an interesting query for your problem would be Discovering which row keys and columns had long lock wait times during the selected period. You can then see if your query is reading those row keys and columns and act accordingly.